Developer Glossary
Plain-English definitions for 58+ terms you'll run into as a developer — from JWTs and CIDR ranges to cron expressions and CSS flexbox. Each entry links to a free tool you can use right away.
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.gitignore
A file that tells Git which files or directories to exclude from version control — typically build output, dependencies, and local configuration/secrets.
.htaccess
A configuration file used by the Apache web server to control settings like redirects, URL rewriting, access restrictions, and caching on a per-directory basis.
A
AES (Advanced Encryption Standard)
A symmetric-key encryption standard used to securely encrypt and decrypt data with a shared secret key. Widely used for protecting data at rest and in transit.
B
Base64
An encoding scheme that represents binary data using 64 printable ASCII characters. Commonly used to embed images in HTML/CSS, encode email attachments, and pass binary data in JSON or URLs.
Bcrypt
A password-hashing function designed to be slow and resistant to brute-force attacks. Includes a built-in salt, making it a common choice for storing user passwords.
Bitwise Operators
Operators (AND, OR, XOR, NOT, shifts) that manipulate the individual bits of integers. Used for flags, masks, performance-sensitive math, and low-level protocol work.
Border Radius
A CSS property that rounds the corners of an element's box. Can take one to four values to control each corner independently.
Box Shadow
A CSS property that adds shadow effects around an element's frame, defined by offset, blur, spread, and color — used for depth and elevation in UI design.
C
chmod / File Permissions
A Unix/Linux command (and the underlying permission model) that controls read, write, and execute access for a file's owner, group, and others — typically expressed as a 3-digit octal number like 755.
CIDR (Classless Inter-Domain Routing)
A notation for specifying IP address ranges, written as an address plus a prefix length (e.g. 192.168.1.0/24). Used for routing and subnet allocation.
Color Models (Hex, RGB, HSL)
Different ways of representing color values — Hex (#RRGGBB), RGB (red/green/blue), and HSL (hue/saturation/lightness) — commonly converted between when working with CSS and design tools.
Content Security Policy (CSP)
An HTTP response header that helps prevent cross-site scripting (XSS) and other injection attacks by restricting which sources of scripts, styles, images, and other resources a page may load.
Checksum / Hash Digest
A fixed-size value computed from a file or message used to verify its integrity — if the data changes, the checksum changes. Common algorithms include MD5, SHA-1, and SHA-256.
Cron Expression
A string of five or six fields (minute, hour, day, month, weekday) that defines a recurring schedule for running tasks — used by cron jobs, CI pipelines, and scheduled functions.
cURL
A command-line tool and library for transferring data with URLs, supporting HTTP, HTTPS, FTP and more — often used to test APIs or copied directly from browser dev tools.
D
Data Size Units (KB, MB, GB...)
Units used to describe digital storage and transfer sizes. Note the distinction between decimal (1 KB = 1000 bytes) and binary (1 KiB = 1024 bytes) definitions, which often causes confusion.
Diff (Difference)
A comparison between two versions of text, code, or structured data that highlights what was added, removed, or changed — fundamental to version control and code review.
E
Environment Variables
Key-value pairs available to a process at runtime, commonly used to configure applications (API keys, database URLs, feature flags) without hardcoding them in source code — often stored in `.env` files.
Exit Code
A small integer returned by a program when it finishes, indicating success (0) or a specific type of failure (non-zero). Used heavily in shell scripting and CI pipelines.
F
Favicon
The small icon associated with a website, shown in browser tabs, bookmarks, and history. Modern sites typically need multiple sizes/formats for different devices and platforms.
Flexbox
A CSS layout model (`display: flex`) for arranging items in a single row or column, with powerful controls for alignment, spacing, and ordering — widely used for responsive UI.
G
GraphQL
A query language and runtime for APIs that lets clients request exactly the data they need, in a single request, using a strongly-typed schema — an alternative to traditional REST endpoints.
H
Hashing (MD5, SHA-1, SHA-256)
A one-way function that converts input data of any size into a fixed-size string. Used for integrity checks, deduplication, and (with proper salting) password storage. MD5/SHA-1 are considered cryptographically broken for security purposes.
HTML Entities
Special codes (like `&`, `<`, `©`) used to represent characters that have special meaning in HTML or that aren't easily typed — preventing markup from being misinterpreted.
HTTP Header
Key-value metadata sent with HTTP requests and responses — covering things like content type, caching, authentication, and security policies.
HTTP Method
The verb in an HTTP request (GET, POST, PUT, PATCH, DELETE, etc.) that indicates the desired action to perform on a resource, central to REST API design.
HTTP Status Code
A 3-digit code returned by a server indicating the result of a request — e.g. 200 (OK), 301 (redirect), 404 (not found), 500 (server error). Grouped into 1xx–5xx ranges by category.
J
JSON (JavaScript Object Notation)
A lightweight, text-based data format for representing structured data as key-value pairs and arrays. The de facto standard for web APIs and configuration files.
JSON Schema
A vocabulary for describing the structure, types, and constraints of JSON data — used to validate payloads, generate documentation, and drive form/UI generation.
JWT (JSON Web Token)
A compact, URL-safe token format consisting of a header, payload, and signature, used to securely transmit claims (e.g. user identity) between parties — common in authentication and authorization.
K
Keyframes (CSS Animation)
A CSS rule (`@keyframes`) that defines intermediate steps in an animation sequence, specifying styles at different points (0%, 50%, 100%) along the timeline.
L
Lorem Ipsum
Placeholder text derived from a classical Latin text, used by designers and developers to fill layouts with realistic-looking content before real copy is available.
M
Markdown
A lightweight markup language using plain-text syntax (headings, lists, links, emphasis) that converts to HTML — widely used for README files, documentation, and chat formatting.
MIME Type
A standardized label (e.g. `application/json`, `image/png`) that identifies the format of a file or data stream, used in HTTP headers and email attachments to tell clients how to handle content.
Mock Data
Artificially generated data that resembles real records (names, emails, addresses, etc.), used for testing, prototyping, and populating UIs before real data is available.
N
Number Base (Binary, Octal, Hex)
A system for representing numbers using a fixed set of digits — binary (base 2), octal (base 8), decimal (base 10), and hexadecimal (base 16) are the most common in computing.
P
Percentage Calculation
Expressing a number as a fraction of 100 — used for everything from discounts and statistics to CSS sizing and progress indicators.
Port (Network Port)
A 16-bit number (0–65535) that identifies a specific process or service on a networked device — e.g. port 443 for HTTPS, port 22 for SSH.
Prime Number
A natural number greater than 1 with no positive divisors other than 1 and itself — fundamental to cryptography (e.g. RSA key generation) and computer science algorithms.
Q
QR Code
A two-dimensional barcode that encodes text, URLs, or other data, readable by smartphone cameras — widely used for sharing links, Wi-Fi credentials, and payment information.
R
Regular Expression (Regex)
A sequence of characters that defines a search pattern, used for matching, validating, and replacing text — supported across nearly every programming language.
robots.txt
A text file placed at a site's root that tells search engine crawlers which pages or sections they may or may not access — part of the Robots Exclusion Protocol.
Roman Numerals
A numeral system from ancient Rome using letters (I, V, X, L, C, D, M) to represent values — still used today for things like clock faces, outlines, and sequels.
RSA
An asymmetric (public-key) cryptography algorithm used for secure data transmission and digital signatures, based on the mathematical difficulty of factoring large prime numbers.
S
Slug (URL Slug)
A URL-friendly version of a string — lowercase, hyphen-separated, with special characters removed — commonly used for blog post URLs and identifiers (e.g. "my-blog-post").
SQL (Structured Query Language)
The standard language for querying and managing relational databases — used to select, insert, update, and delete data, and to define schema structure.
Subnet Mask
A 32-bit value that divides an IP address into network and host portions, determining which addresses belong to the same local network.
T
TOML
A configuration file format (Tom's Obvious, Minimal Language) designed to be easy to read due to clear semantics — used by tools like Cargo (Rust) and pyproject.toml (Python).
TOTP (Time-based One-Time Password)
A short-lived numeric code generated from a shared secret and the current time, used as a second authentication factor (2FA) in apps like Google Authenticator.
U
Unicode / Character Encoding
A universal character encoding standard that assigns a unique code point to virtually every character across all writing systems. UTF-8 is the most common encoding used to represent Unicode on the web.
Unix Timestamp
The number of seconds (or milliseconds) since January 1, 1970 UTC (the "Unix epoch") — a compact, timezone-independent way to represent a point in time.
URL Encoding (Percent-Encoding)
A method of encoding characters in a URL using a `%` followed by two hexadecimal digits, used to represent characters that aren't allowed or have special meaning in URLs (spaces, `&`, `?`, etc.).
User Agent
A string sent by browsers and HTTP clients that identifies the application, operating system, vendor, and version — used for analytics, feature detection, and compatibility checks.
UUID (Universally Unique Identifier)
A 128-bit identifier, usually shown as 32 hex digits in 5 groups (e.g. `123e4567-e89b-12d3-a456-426614174000`), designed to be unique across systems without coordination.
W
WCAG Contrast Ratio
A measure of the difference in luminance between text and background colors, used by the Web Content Accessibility Guidelines (WCAG) to ensure text is readable — AA and AAA levels define minimum ratios.
X
XML (Extensible Markup Language)
A markup language that defines rules for encoding documents in a format that is both human- and machine-readable, using custom tags — still widely used in enterprise systems, configs, and feeds.
Y
YAML
A human-readable data serialization format that uses indentation instead of brackets/braces — commonly used for configuration files (CI pipelines, Kubernetes, Docker Compose).