Glossary

AES-256 encryption PDF

Advanced Encryption Standard with a 256-bit key: a cipher standardised by NIST and ISO and used in secure web connections, disk encryption and password-protected PDFs. Without the password the file cannot practically be opened, so the real weak point is a short or guessable password, not the encryption itself.

Tools: PDF Protect, PDF UnlockGuide: How to Password-Protect a PDF (and What PDF Passwords Can and Can't Do)

Alpha channel Images

The part of an image that stores transparency for each pixel, from fully see-through to fully solid. PNG, WebP and AVIF can store it; JPEG cannot, so transparent areas turn into a solid colour, often white or black, when you convert to JPG. Logos and cut-out product photos usually need it.

Tools: Background Remover, PNG to JPG, AVIF to PNGGuides: How to Remove a Background from a Photo: Tips for Clean Results, JPG vs PNG vs WebP: Which Image Format Should You Use?

APR Finance

Annual percentage rate: the yearly cost of borrowing, including interest and certain fees, shown as a percentage. Because it is worked out on the reducing balance and includes charges, it is the fairest number for comparing loans and credit cards. Which fees must be included depends on local rules.

Tools: Loan Calculator, Mortgage CalculatorGuide: Flat Rate vs Reducing Balance Loans: Why the Same Rate Costs Different Amounts

ASCII Text

American Standard Code for Information Interchange: a character code from 1963 with 128 characters, numbered 0 to 127, covering English letters, digits, punctuation and control codes. A is 65 and a is 97. It has no accented letters or other scripts, which is why Unicode replaced it, though UTF-8 keeps the same codes for these characters.

Tools: ASCII Converter, Text to Binary, Text to Hex

Aspect ratio Images

The shape of an image or screen, written as width to height. Common ratios are 16:9 for video and most screens (1920 × 1080), 4:3 for most phone cameras, 1:1 for square posts and 4:5 for portrait social posts. Changing the ratio means cropping or adding borders, or the picture looks stretched.

Tools: Image Cropper, Ratio Calculator, Instagram Image ResizerGuide: How to Resize and Compress Images for Websites, Email and Forms

AVIF Images

AV1 Image File Format: a modern image format based on the AV1 video codec. It supports transparency and HDR and often makes files smaller than WebP or JPEG at the same visual quality. Current browsers open it, but some older apps, email programs and upload forms still don't.

Tools: AVIF to JPG, AVIF to PNGGuide: JPG vs PNG vs WebP: Which Image Format Should You Use?

Bandwidth Networks

The most data a connection can carry each second, usually given in megabits per second (Mbps). Divide by 8 to get megabytes per second, so a 100 Mbps line downloads at most about 12.5 MB/s. Speed tests measure download and upload separately because many connections are much faster one way.

Tools: Internet Speed Test, Network Speed Calculator, Download Speed TestGuide: How to Test Your Internet Speed Properly (and What the Numbers Mean)

Base64 Developer

A way to write any data, such as an image or a file, using only 64 safe text characters (A–Z, a–z, 0–9, + and /), with = as padding. It makes data about 33% larger and is not encryption, since anyone can decode it. It is used in email attachments, data URIs and JWTs.

Tools: Base64 Encoder / Decoder, Image to Base64Guide: Base64, URL Encoding and JWTs: What They Are (and Why They're Not Encryption)

Blacklist (DNSBL) Email

A DNS-based blocklist: a public list of IP addresses and domains that have sent spam, such as Spamhaus or SpamCop. Mail servers check it for incoming messages and may reject mail from listed addresses. Listings usually follow a hacked account, an open relay or a bad mailing list, and most lists accept delisting requests once the cause is fixed.

Tools: IP Blacklist Checker, Reverse DNS Lookup, Email Header AnalyzerGuide: SPF, DKIM and DMARC Explained: How to Stop Email Spoofing

BMI Health

Body mass index: weight in kilograms divided by height in metres squared, so a 70 kg adult who is 1.75 m tall has a BMI of 22.9. The WHO adult ranges are under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight and 30 or more obesity. It is a screening estimate that can't tell muscle from fat.

Tools: BMI Calculator, Calorie CalculatorGuide: BMI Explained: How It's Calculated, What It Tells You and Its Limits

BMR Health

Basal metabolic rate: an estimate of the calories your body burns at complete rest to keep breathing, circulating blood and staying warm. Calculators usually use the Mifflin–St Jeor formula, which gives about 1,720 kcal a day for a 30-year-old man weighing 75 kg at 178 cm. Real values vary from person to person.

Tools: Calorie Calculator

Brute-force attack Security

Trying every possible password or key until one works. Computers can test billions of guesses a second against a stolen password hash, so short passwords fall quickly, while each extra random character multiplies the work. Long random passwords, slow password hashing and limits on login attempts make brute force impractical.

Tools: Password Generator, Hash GeneratorGuide: How to Create a Strong Password in 2026

Character count Text

The number of characters in a text, usually shown with and without spaces. Many places set limits: a standard post on X allows 280 characters, and one SMS holds 160 plain characters, or only 70 if it contains emoji or letters outside the basic Latin set. Longer text messages are split into parts.

Tools: Character Counter, Word CounterGuide: Word and Character Limits for Essays, Social Posts and Search Results

CIDR Networks

Classless Inter-Domain Routing: writing an IP range as an address, a slash and the number of fixed network bits. 192.168.1.0/24 covers 256 addresses (192.168.1.0 to 192.168.1.255), /16 covers 65,536 and /32 is a single address. Each step down in the number doubles the size of the range.

Tools: CIDR Calculator, Subnet CalculatorGuide: Subnetting Made Simple: CIDR, Subnet Masks and Usable Hosts

CMYK Images

Cyan, magenta, yellow and key (black): the four inks used in colour printing, each given as a percentage. Printing is subtractive, so more ink makes a darker colour. Some bright screen colours, especially vivid greens and blues, can't be printed exactly, so designs for print are often checked in CMYK first.

Tools: Color Converter, Image Color Picker

Compound interest Finance

Interest calculated on the original amount plus the interest already added, so growth speeds up over time. 1,000 at 5% a year, compounded annually, becomes 1,628.89 after 10 years, compared with 1,500 under simple interest, which is paid only on the original amount. More frequent compounding adds slightly more.

Tools: Compound Interest Calculator, Simple Interest CalculatorGuide: How Compound Interest Works (With Examples You Can Check)

Cron expression Developer

A schedule written as five fields: minute, hour, day of month, month and day of week. For example, 0 9 * * 1-5 means 09:00 every Monday to Friday, and */15 * * * * means every 15 minutes. Servers often run cron in UTC, so check the time zone before relying on it.

Tools: Cron Expression Explainer, Cron Expression GeneratorGuide: Cron Expressions Explained: Read and Write Schedules Without Mistakes

Cross-reference table PDF

The index inside a PDF that records where each object, such as a page, font or image, starts in the file, so a reader can jump straight to it. It sits near the end of the file. If a download is cut off or the file is badly edited, the table breaks and readers show errors or must rebuild it.

Tools: PDF Repair & Checker

CSV Developer

Comma-separated values: a plain-text table where each line is a row and commas separate the columns. Values that contain commas or line breaks are wrapped in double quotes. In countries that use a comma as the decimal mark, spreadsheet programs often use semicolons instead, a common cause of import problems.

Tools: CSV to JSON Converter, JSON to CSV Converter, PDF to CSV

DKIM Email

DomainKeys Identified Mail: a digital signature that the sending server adds to each email's header. The receiving server fetches the public key from a DNS TXT record at selector._domainkey.yourdomain and checks that the message was signed for that domain and not changed on the way.

Tools: DKIM Record Generator, SPF, DKIM & DMARC Checker, Email Header AnalyzerGuide: SPF, DKIM and DMARC Explained: How to Stop Email Spoofing

DMARC Email

Domain-based Message Authentication, Reporting and Conformance: a DNS TXT record at _dmarc.yourdomain that tells receiving servers what to do when mail using your domain in the From address passes neither SPF nor DKIM for that domain: deliver it anyway (p=none), send it to spam (quarantine) or refuse it (reject). It can also request reports.

Tools: DMARC Generator, SPF, DKIM & DMARC CheckerGuide: SPF, DKIM and DMARC Explained: How to Stop Email Spoofing

DNS Networks

Domain Name System: the internet's directory that turns names like example.com into the IP addresses computers use to connect. Your device asks a resolver, usually run by your provider or a public service such as Cloudflare or Google, which looks up the answer and keeps it for a while.

Tools: DNS Lookup, DNS Record Checker, DNS Speed TestGuide: DNS Records Explained: A, AAAA, CNAME, MX, TXT and More

DNS propagation Networks

The time it takes for a DNS change to be seen everywhere. Nothing is actually pushed out: resolvers keep the old answer in their cache until its TTL runs out, then fetch the new one. Most changes appear within minutes to a few hours, but name server changes can take up to about 48 hours.

Tools: DNS Propagation Checker, DNS LookupGuide: DNS Records Explained: A, AAAA, CNAME, MX, TXT and More

DNS record Networks

One entry in a domain's DNS settings. The common types are A (IPv4 address), AAAA (IPv6 address), CNAME (an alias pointing to another name), MX (mail servers), TXT (text such as SPF or verification codes), NS (the domain's name servers) and PTR (the name for an IP address, used for reverse DNS).

Tools: DNS Lookup, DNS Record Checker, TXT Record CheckerGuide: DNS Records Explained: A, AAAA, CNAME, MX, TXT and More

DPI Images

Dots per inch: how densely a printer lays down dots, and the print-size setting stored inside an image file. Changing the setting doesn't alter the pixels, only how large the picture prints: at 300 DPI, the usual target for photos, 3000 pixels print 10 inches (25.4 cm) wide. PPI (pixels per inch) means much the same for screens.

Tools: Image DPI Converter, Passport Photo Maker, Image ResizerGuide: How to Make a Passport Photo at Home (and Print It at the Right Size)

Dynamic QR code QR codes

A QR code that holds a short link to a service, which then redirects to the real destination. You can change the destination and count scans without reprinting, but the code stops working if the service closes or the subscription ends. A static QR code stores the content itself and keeps working for good.

Tools: QR Code Generator, Redirect CheckerGuide: QR Codes: How They Work, What They Hold and How to Make One That Scans

Email header Email

The hidden part of an email that records its journey and checks: the sender, every server it passed through (the Received lines, read from the bottom up), timestamps, and the SPF, DKIM and DMARC results. Most mail apps show it under an option such as Show original or View message source.

Tools: Email Header AnalyzerGuide: SPF, DKIM and DMARC Explained: How to Stop Email Spoofing

Error correction level QR codes

How much of a QR code can be damaged or covered and still scan. There are four levels: L restores about 7% of the data, M about 15%, Q about 25% and H about 30%. Higher levels make the code denser, but level H makes it safer to put a logo in the middle.

Tools: QR Code Generator, QR Code with Logo, QR Color GeneratorGuide: QR Codes: How They Work, What They Hold and How to Make One That Scans

EXIF Images

Exchangeable Image File Format: the hidden details a camera or phone saves inside a photo, such as the date and time, device model, exposure settings and often the exact GPS location. Many social networks remove it when you upload, but email and file transfers usually keep it, so check before sharing.

Tools: Image Metadata Viewer, Remove Image MetadataGuide: What Is EXIF Data? See and Remove Hidden Photo Details Before You Share

Favicon Images

The small icon shown in a browser tab, bookmark or home-screen shortcut for a website. The classic format is an .ico file that holds several sizes, such as 16×16, 32×32 and 48×48 pixels, and modern sites add PNG versions, such as a 180×180 icon for iPhone home screens.

Tools: Favicon Generator, ICO Converter

Flat rate interest Finance

Interest charged on the full amount borrowed for the whole term, even as you pay it back. A 5% flat rate on 10,000 over 3 years costs 1,500, which is roughly 9.3% on a reducing-balance basis, where interest is charged only on what you still owe. Compare loans by their APR instead.

Tools: Loan CalculatorGuide: Flat Rate vs Reducing Balance Loans: Why the Same Rate Costs Different Amounts

Flattening PDF

Merging a PDF's form answers, comments, stamps and drawn signatures into the page itself, so they can no longer be edited or removed and look the same in every reader. It is often done before sending a completed form or contract. Keep an unflattened copy in case you need to change the answers.

Tools: PDF Flatten, PDF Form Filler

Hash Security

A fixed-length fingerprint made from any data by a one-way function. The same input always gives the same hash, and even a one-letter change gives a completely different one, so hashes are used to check downloads and store passwords. Older algorithms such as MD5 and SHA-1 are broken and should not be used for security.

Tools: Hash GeneratorGuide: What Is a Hash? SHA-256 and File Checks Explained Simply

HEIC Images

The photo format iPhones and iPads use by default since iOS 11: a HEIF image compressed with HEVC (H.265) video coding. A HEIC photo is often about half the size of a JPEG of similar quality, but Windows, many websites and upload forms don't always open it, so it is often converted to JPG.

Tools: HEIC to JPG, HEIC to PNGGuide: HEIC vs JPG: What iPhone Photos Are Saved As, and When to Convert

HEX colour code Images

A colour written as # followed by six hexadecimal digits: two each for red, green and blue, from 00 to FF. #FF0000 is pure red, #FFFFFF white and #000000 black, giving about 16.7 million colours in all. A three-digit short form such as #F00 and an eight-digit form with transparency also exist.

Tools: Color Converter, Image Color Picker, Color Palette Generator

HSTS Websites

HTTP Strict Transport Security: a response header that tells browsers to use only HTTPS for a site for a set time, for example Strict-Transport-Security: max-age=31536000 (one year). It protects visitors from attackers who try to downgrade connections to plain HTTP. Check that every subdomain works over HTTPS before adding includeSubDomains.

Tools: HTTP Headers Checker, SSL CheckerGuide: How SSL/TLS Certificates Work, and How to Check One Before It Expires

HTTP status code Websites

The three-digit number a web server sends with every response. 2xx means success (200 OK), 3xx a redirect (301, 302), 4xx a problem with the request (404 Not Found, 403 Forbidden) and 5xx a server error (500, 503 Service Unavailable). 1xx codes are informational and rarely seen.

Tools: HTTP Status Checker, Redirect Checker, Website Uptime Checker

Imperial units Units

The system of inches, feet, miles, pounds, stones, pints and gallons defined in the UK, where it is still used alongside metric. US customary units share most names but differ for volume: a UK pint is 568 ml and a US pint 473 ml, and a UK gallon is 4.546 litres against 3.785 in the US. Both define the inch as exactly 2.54 cm.

Tools: Volume Converter, Length Converter, Weight ConverterGuide: Metric vs Imperial Units: Exact Conversions You Can Rely On

IP address Networks

Internet Protocol address: the number that identifies a device on a network so data can reach it. The public IP address that websites see is assigned by your internet provider and is often shared by everyone on your home network. It reveals your rough location and provider, but not your street address.

Tools: IP Address Checker, Domain IP Lookup, IPv4 CalculatorGuide: What Is My IP Address? IPv4 vs IPv6 Explained

IPv4 Networks

Internet Protocol version 4: 32-bit addresses written as four numbers from 0 to 255, such as 192.0.2.10. There are only about 4.3 billion of them, not enough for every device, so providers share addresses using NAT and networks are gradually adding IPv6.

Tools: IPv4 Calculator, Subnet Calculator, IP to Decimal ConverterGuides: What Is My IP Address? IPv4 vs IPv6 Explained, Subnetting Made Simple: CIDR, Subnet Masks and Usable Hosts

IPv6 Networks

Internet Protocol version 6: 128-bit addresses written as eight groups of hexadecimal digits separated by colons, such as 2001:db8::1, where :: stands for a run of zero groups. There are enough addresses to give every device its own public one, and many mobile and home networks already use IPv6 alongside IPv4.

Tools: IPv6 Calculator, IPv4/IPv6 Compatibility Checker, IP Address CheckerGuide: What Is My IP Address? IPv4 vs IPv6 Explained

Jitter Networks

The variation in latency from one moment to the next, measured in milliseconds. A connection whose ping jumps between 20 and 90 ms has high jitter, which causes choppy video calls and lag in games even when the average looks fine. Under about 30 ms is usually fine for calls.

Tools: Internet Speed Test, Ping TestGuide: How to Test Your Internet Speed Properly (and What the Numbers Mean)

JPEG Images

Joint Photographic Experts Group, the committee behind the most widely used photo format (.jpg or .jpeg). It uses lossy compression, so photos are small but lose a little detail every time they are saved again. It has no transparency, which makes PNG a better choice for logos, screenshots and text.

Tools: Image Compressor, PNG to JPG, JPG to PNGGuide: JPG vs PNG vs WebP: Which Image Format Should You Use?

JSON Developer

JavaScript Object Notation: a text format for data made of objects in curly braces, lists in square brackets, and strings, numbers, true, false and null. It is the usual format for web APIs and settings files. Keys and strings need double quotes, and comments and trailing commas are not allowed.

Tools: JSON Formatter, JSON Validator, JSON MinifierGuide: JSON Explained: How to Format, Validate and Fix Common Errors

JWT Developer

JSON Web Token: a compact login or access token made of three Base64url parts separated by dots: a header, a payload of claims such as the user ID and expiry time (exp), and a signature. Anyone can read the payload, so it must not hold secrets, and servers must verify the signature before trusting it.

Tools: JWT Decoder, Base64 Encoder / DecoderGuide: Base64, URL Encoding and JWTs: What They Are (and Why They're Not Encryption)

Kibibyte Units

A kibibyte (KiB) is 1,024 bytes, while a kilobyte (kB) is 1,000 bytes. The same split applies to MiB and MB, and GiB and GB. Drive makers use decimal units, but Windows counts in binary and still labels the result GB, which is why a 1 TB drive shows as about 931 GB.

Tools: Data Storage Converter, Network Speed Calculator

Lossless compression Images

Making a file smaller without losing any information, so the decompressed result is identical to the original. PNG, lossless WebP and ZIP work this way. The savings are usually modest for photos, but it suits screenshots, logos, text and anything that will be edited again.

Tools: Image Compressor, JPG to PNG, HEIC to PNGGuides: JPG vs PNG vs WebP: Which Image Format Should You Use?, How to Resize and Compress Images for Websites, Email and Forms

Lossy compression Images

Making a file much smaller by permanently discarding detail that people are unlikely to notice. JPEG, lossy WebP, AVIF and MP3 work this way, usually with a quality setting: for photos, around 75–85 out of 100 often looks almost identical to the original. Saving the same file again and again adds more loss.

Tools: Image Compressor, JPG to WebP, Compress PDFGuides: How to Resize and Compress Images for Websites, Email and Forms, How to Make a PDF Smaller (and Why Some PDFs Won't Shrink)

MAC address Networks

Media Access Control address: the hardware ID of a network card, written as six pairs of hexadecimal digits, such as 00:1A:2B:3C:4D:5E. The first half, the OUI, identifies the maker and is assigned by the IEEE. Phones and laptops now often use a random MAC address for each Wi-Fi network, for privacy.

Tools: MAC Address Lookup, MAC Address Formatter

Markup Finance

Profit as a percentage of cost. An item that costs 80 and sells for 100 has a markup of 25%. Markup is always higher than margin for the same sale, so mixing them up leads to prices that are too low: a 25% markup is only a 20% margin.

Tools: Markup Calculator, Profit Margin CalculatorGuide: Profit Margin vs Markup: Formulas, Examples and Common Mistakes

Metadata PDF

Data about a file rather than its content, such as the author, title, software used, creation and edit dates and, for photos, the camera and location. It travels with the file and can reveal more than you intend, such as a colleague's name in a PDF, so it is worth checking before sharing.

Tools: PDF Metadata Remover, Image Metadata Viewer, Remove Image MetadataGuide: What Is EXIF Data? See and Remove Hidden Photo Details Before You Share

Metric system Units

The decimal system of measurement, formally the International System of Units (SI), used by almost every country. Units scale by powers of ten with prefixes such as kilo (1,000), centi (1/100) and milli (1/1,000), so 1 km is 1,000 m and 1 litre is 1,000 ml. Its base units include the metre, kilogram and second.

Tools: Length Converter, Weight Converter, Temperature ConverterGuide: Metric vs Imperial Units: Exact Conversions You Can Rely On

Mid-market rate Finance

The midpoint between the buying and selling prices of two currencies on the wholesale market, often called the real exchange rate. It is the fairest reference, but banks, cards and exchange shops usually add a margin on top, so the gap between their rate and the mid-market rate is a hidden fee.

Tools: Currency ConverterGuide: Exchange Rates Explained: Mid-Market Rates, Spreads and Card Fees

MX record Email

Mail exchanger record: the DNS entry that says which servers accept email for a domain. Each has a priority number, and lower numbers are tried first, for example 10 for the main server and 20 for a backup. A missing or wrong MX record means incoming email bounces or goes astray.

Tools: MX Record Checker, DNS Lookup, DNS Record CheckerGuides: DNS Records Explained: A, AAAA, CNAME, MX, TXT and More, SPF, DKIM and DMARC Explained: How to Stop Email Spoofing

OCR PDF

Optical character recognition: software that reads the letters in a picture, such as a scanned page or a photo of a document, and turns them into text you can search, copy and edit. Accuracy depends on the image: sharp, straight scans at about 300 DPI in a clear font work best.

Tools: PDF OCR, Image to Text (OCR)Guide: How to Turn a Scanned PDF Into Searchable, Editable Text (OCR)

Overtime rate Finance

The higher hourly pay for hours worked beyond the normal limit, written as a multiple of the usual rate. Time and a half (1.5×) turns 20 an hour into 30, and double time (2×) turns it into 40. When overtime starts and which rate applies depend on local law and your contract.

Tools: Overtime Calculator, Salary Calculator, Time CalculatorGuide: How to Calculate Overtime Pay: Time and a Half, Double Time and Salaried Workers

Password entropy Security

A measure of how hard a password is to guess, in bits; each extra bit doubles the number of guesses needed. A random password scores its length times log2 of the number of possible characters, so 12 characters from all 94 printable keyboard symbols give about 79 bits. Passwords people make up score far lower than the formula suggests.

Tools: Password GeneratorGuide: How to Create a Strong Password in 2026

PDF PDF

Portable Document Format: a file type created by Adobe in 1993 and now the open standard ISO 32000. A PDF keeps its fonts, images and layout, so it looks the same on any device and prints as intended, which is why it is used for contracts, forms, invoices and reports.

Tools: PDF Merge, Compress PDF, JPG to PDFGuides: How to Make a PDF Smaller (and Why Some PDFs Won't Shrink), How to Merge, Split and Rotate PDF Files Without Uploading Them

PDF/A PDF

A version of PDF made for long-term archiving, defined in ISO 19005. It embeds every font and colour profile and forbids features that may stop working later, such as encryption, external content, audio, video and JavaScript. Courts, government offices and archives often ask for documents in this format.

Ping Networks

A test that measures latency: the round-trip time, in milliseconds, for a small message to reach a server and come back. Under about 50 ms feels quick for browsing, calls and games, while older satellite links often exceed 500 ms. Ping also shows whether a server is reachable at all.

Tools: Ping Test, Internet Speed TestGuide: How to Test Your Internet Speed Properly (and What the Numbers Mean)

PNG Images

Portable Network Graphics: an image format with lossless compression and full transparency. It keeps text, lines and flat colours perfectly sharp, which suits screenshots, logos and diagrams, but a photo saved as PNG is often several times bigger than the same photo saved as JPEG.

Tools: JPG to PNG, PNG to JPG, Image CompressorGuide: JPG vs PNG vs WebP: Which Image Format Should You Use?

Port Networks

A number from 0 to 65535 that directs TCP or UDP traffic to the right service on a device. Common ones are 80 for HTTP, 443 for HTTPS, 22 for SSH, 25 for email between servers and 3389 for Remote Desktop. An open port has a program listening on it, and a firewall can block it.

Tools: Port Checker, Ping TestGuide: How to Check if a Port Is Open (and What the Results Mean)

Private IP address Networks

An address reserved for local networks that the internet can't reach directly: 10.0.0.0–10.255.255.255, 172.16.0.0–172.31.255.255 and 192.168.0.0–192.168.255.255. Your router gives devices at home addresses such as 192.168.1.20 and shares one public IP address between them using NAT.

Tools: IPv4 Calculator, IP Address Checker, Subnet CalculatorGuide: What Is My IP Address? IPv4 vs IPv6 Explained

Profit margin Finance

Profit as a percentage of the selling price. An item that costs 60 and sells for 100 makes 40 profit, a 40% margin, which is the same as a 66.7% markup. Gross margin counts only the cost of the goods, while net margin also subtracts running costs, tax and interest.

Tools: Profit Margin Calculator, Markup CalculatorGuide: Profit Margin vs Markup: Formulas, Examples and Common Mistakes

QR code QR codes

Quick Response code: a square barcode invented by Denso Wave in 1994 and standardised as ISO/IEC 18004. A phone camera reads it to open a link, join Wi-Fi, save a contact or show text. The largest version holds up to 7,089 digits or 2,953 bytes, but shorter content gives a simpler code that scans more easily.

Tools: QR Code Generator, QR Code Scanner, QR Code DecoderGuides: QR Codes: How They Work, What They Hold and How to Make One That Scans, How to Share Your Wi-Fi With a QR Code (iPhone and Android)

Redirect Websites

An instruction that sends visitors and search engines from one web address to another. A 301 means moved permanently, so search engines show the new address instead of the old one; a 302 means temporary, and 307 and 308 are stricter versions of 302 and 301 that keep the request method. Long chains of redirects slow pages down.

Tools: Redirect Checker, HTTP Status Checker

Regex Developer

Regular expression: a pattern for finding or checking text. For example, \d{3} matches any three digits, ^ and $ mark the start and end, and [A-Z]+ matches one or more capital letters. Regex is used for search and replace, form validation and searching logs, and details vary slightly between programming languages.

Tools: Regex Tester, Find & Replace, Text ExtractorGuide: Regular Expressions for Beginners: A Practical Guide With Examples

Resolution Images

The size of an image in pixels, written as width × height, such as 1920 × 1080 (Full HD, about 2.1 megapixels) or 4000 × 3000 (12 megapixels). More pixels allow bigger prints and tighter crops. Enlarging a small image adds pixels but not detail, so the result looks soft.

Tools: Image Resizer, Image Cropper, Image DPI ConverterGuide: How to Resize and Compress Images for Websites, Email and Forms

Reverse DNS Networks

Looking up the host name for an IP address, the opposite of a normal DNS lookup, using a PTR record set by whoever controls the IP range. Mail servers are expected to have one that matches their forward DNS; without it, many receiving servers treat their email as suspicious or reject it.

Tools: Reverse DNS Lookup, IP Blacklist CheckerGuide: DNS Records Explained: A, AAAA, CNAME, MX, TXT and More

RGB Images

Red, green and blue: the three colours of light that screens mix to make every other colour, each usually given from 0 to 255. rgb(255, 165, 0) is orange and rgb(0, 0, 0) is black. HSL describes the same colours as hue, saturation and lightness, which makes lighter and darker shades easier to pick.

Tools: Color Converter, Image Color Picker, Color Palette Generator

SHA-256 Security

A widely used hash function from the SHA-2 family, published by the US standards body NIST. It turns any input into 256 bits, shown as 64 hexadecimal characters, and no practical way is known to reverse it or to find two inputs with the same hash. It protects software downloads, TLS certificates and Bitcoin.

Tools: Hash GeneratorGuide: What Is a Hash? SHA-256 and File Checks Explained Simply

SPF Email

Sender Policy Framework: a DNS TXT record listing the servers allowed to send email for a domain, such as v=spf1 include:_spf.google.com ~all. Receiving servers compare the sending server against it. A domain can have only one SPF record, and checking it may take no more than 10 DNS lookups.

Tools: SPF, DKIM & DMARC Checker, SPF Record Generator, TXT Record CheckerGuide: SPF, DKIM and DMARC Explained: How to Stop Email Spoofing

SSL/TLS certificate Websites

A file that proves a website's identity and lets browsers set up an encrypted HTTPS connection (TLS replaced SSL, but the old name stuck). A certificate authority such as Let's Encrypt or DigiCert issues it after checking that you control the domain. Public certificates now last at most 200 days, falling to 47 days by 2029, so automatic renewal matters.

Tools: SSL Checker, SSL Certificate Expiry CheckerGuide: How SSL/TLS Certificates Work, and How to Check One Before It Expires

Subnet mask Networks

A number that shows which part of an IPv4 address identifies the network and which part the device. 255.255.255.0, the same as /24, means the first three numbers are the network, leaving 254 usable addresses for devices. Devices in the same subnet talk directly; traffic to other subnets goes through a router.

Tools: Subnet Calculator, CIDR Calculator, IPv4 CalculatorGuide: Subnetting Made Simple: CIDR, Subnet Masks and Usable Hosts

SVG Images

Scalable Vector Graphics: an image format that describes shapes, lines and text in XML code instead of pixels, so it stays sharp at any size. It suits logos, icons and charts rather than photos. Some websites and apps don't accept SVG uploads, so it is often converted to PNG.

Tools: SVG to PNG, SVG to JPG, Favicon Generator

TDEE Health

Total daily energy expenditure: an estimate of all the calories you burn in a day, found by multiplying BMR by an activity factor from about 1.2 (little exercise) to 1.9 (very hard exercise or physical work). Eating below it tends to lead to weight loss and above it to weight gain, though real needs vary.

Tools: Calorie Calculator, BMI Calculator

TTL Networks

Time to live: how long, in seconds, resolvers may keep a copy of a DNS record before asking again, so 3600 means one hour and 300 means five minutes. Lowering the TTL well before a planned change makes the switch take effect faster. In IP packets, TTL instead limits how many routers a packet can pass through.

Tools: DNS Lookup, DNS Propagation CheckerGuide: DNS Records Explained: A, AAAA, CNAME, MX, TXT and More

Two-factor authentication Security

Signing in with two different kinds of proof, usually a password plus a code from an authenticator app, a text message or a security key. A stolen password alone is then not enough. App codes and hardware keys are safer than SMS codes, which can be intercepted or moved to another SIM card.

Tools: Password GeneratorGuide: How to Create a Strong Password in 2026

Unicode Text

The worldwide standard that gives every character, from the letters of every script to symbols and emoji, a unique number called a code point, written like U+0041 for A or U+1F600 for 😀. It covers more than 150,000 characters. UTF-8 and UTF-16 are the encodings that store these numbers as bytes.

Tools: Unicode Converter, Text to Hex, ASCII Converter

Unix timestamp Developer

The number of seconds since 00:00:00 UTC on 1 January 1970, which computers use to store a moment in time regardless of time zone. For example, 1700000000 is 14 November 2023 at 22:13:20 UTC. JavaScript counts in milliseconds (13 digits), and old 32-bit systems overflow on 19 January 2038.

Tools: Timestamp ConverterGuide: Unix Timestamps Explained: Seconds, Milliseconds and Time Zones

Uptime Websites

The share of time a website or server is working, given as a percentage. 99.9% uptime still allows about 8 hours 46 minutes of downtime a year, 99.99% about 53 minutes and 99.999% about 5 minutes. Uptime monitors check a site every few minutes and alert you when it stops answering.

Tools: Website Uptime Checker, Ping Test, Website Response Time Checker

URL encoding Developer

Also called percent-encoding: replacing characters that aren't allowed in a web address, or that have a special meaning there, with % and their UTF-8 byte values in hexadecimal. A space becomes %20 (or + in form data), & becomes %26 and é becomes %C3%A9.

Tools: URL Encoder / DecoderGuide: Base64, URL Encoding and JWTs: What They Are (and Why They're Not Encryption)

User agent Developer

The text a browser or app sends with each request to say what it is, such as Mozilla/5.0 (Windows NT 10.0; Win64; x64) followed by browser details. Websites use it to recognise devices and bots. It is easy to fake, and modern browsers share less detail in it for privacy.

Tools: User-Agent Checker

UTF-8 Text

The most common way to store Unicode text as bytes, used by about 99% of websites. English letters and digits take one byte, the same as ASCII, while accented letters take two, most Chinese, Japanese and Korean characters three, and most emoji four. Garbled text such as é instead of é usually means UTF-8 was read with the wrong encoding.

Tools: Unicode Converter, Text to Hex, Text to Binary

UUID Developer

Universally unique identifier: a 128-bit ID written as 36 characters, such as 123e4567-e89b-42d3-a456-426614174000, which Microsoft calls a GUID. Version 4 is random, and version 7 starts with a timestamp so IDs sort by creation time, which suits databases. Two random UUIDs matching is so unlikely it can be ignored.

Tools: UUID Generator

VAT Finance

Value added tax: a tax on most goods and services, collected at each stage of sale but paid in the end by the consumer; it is called GST in many countries, and US sales tax differs by being charged only at the final sale. Rates vary: 5% in the UAE, 10% GST in Australia, 20% in the UK. To remove 20% VAT, divide by 1.2 rather than subtracting 20%.

Tools: VAT Calculator, Percentage CalculatorGuide: VAT, GST and Sales Tax Explained: How to Add or Remove Tax From a Price

vCard QR codes

The standard file format for contact details (.vcf), holding a name, phone numbers, email, company, address and website. Phones import it directly, so a vCard QR code on a business card lets people save your details in one scan. Version 3.0 is the most widely supported, and 4.0 (RFC 6350) is the current standard.

Tools: vCard QR Generator, QR Code GeneratorGuide: QR Codes: How They Work, What They Hold and How to Make One That Scans

WebP Images

An image format from Google that supports lossy and lossless compression, transparency and animation. Google reports lossy WebP files as 25–34% smaller than comparable JPEGs, and lossless ones as 26% smaller than PNGs. All current browsers show it, but some older software doesn't, so it is often converted to JPG.

Tools: JPG to WebP, WebP to JPG, JPG / PNG / WebP ConverterGuide: JPG vs PNG vs WebP: Which Image Format Should You Use?

WHOIS Networks

A lookup that shows which registrar a domain is registered with, when it was registered and when it expires, plus its name servers. For generic domains such as .com, ICANN replaced WHOIS with the newer RDAP in 2025, and owner names and emails are now usually hidden under privacy laws such as GDPR.

Tools: WHOIS Lookup, Domain IP Lookup, ASN Lookup

YAML Developer

YAML Ain't Markup Language: a human-friendly data format that uses indentation instead of brackets, common in configuration files for Docker Compose, Kubernetes and GitHub Actions. Indents must use spaces, not tabs, and a wrong indent changes the meaning. Since version 1.2, YAML is a superset of JSON, so almost any JSON file is also valid YAML.

Tools: YAML to JSON Converter, YAML Formatter & ValidatorGuide: JSON Explained: How to Format, Validate and Fix Common Errors