Plain-English developer glossary

45 terms that come up constantly in software work, defined in a sentence or two without assuming you already know the answer. Written for people reading job descriptions, supplier proposals and documentation.

API

An agreed way for two programs to exchange data — a set of addresses you can call, with defined inputs and outputs. Most integrations are a series of HTTP calls to someone else's API.

Back end

The part of a system running on a server: business logic, databases, authentication and scheduled jobs. Nobody sees it directly; everything depends on it.

Branch

A parallel line of development in version control. Work happens on a branch and is merged back when it is ready, so unfinished changes never block the main line.

Cache

A copy of data kept somewhere faster than the original. Caching is the first answer to most performance problems and the cause of most stale-content bugs.

CDN

A content delivery network — servers spread worldwide that keep copies of your files and serve each visitor from the nearest one, cutting latency dramatically.

CI/CD

Continuous integration and delivery: automation that tests every change and deploys it when the tests pass, so releases become routine rather than events.

Core Web Vitals

Google's three page-experience measurements: Largest Contentful Paint, Interaction to Next Paint and Cumulative Layout Shift.

CORS

Cross-origin resource sharing. The browser rule that stops a page on one domain from reading responses from another unless that other server allows it.

CRUD

Create, read, update, delete — the four operations most data-backed applications are built from.

CSS

The language that controls how a page looks: colour, spacing, typography and layout.

Database

An organised store of data with a query language. Relational databases such as PostgreSQL use tables and SQL; document databases store JSON-like records.

Dependency

An external library your project relies on. Every dependency is code you did not write but are responsible for, including its security problems.

DNS

The system that translates a domain name into the IP address of a server. Changes propagate in minutes to hours.

Environment variable

A configuration value supplied to a program from outside its code — the correct place for API keys and database passwords.

Framework

A structured foundation that supplies conventions and common functionality so you write less plumbing. React, Django and Rails are frameworks.

Front end

Everything running in the visitor's browser: markup, styles and client-side scripts.

Git

The version control system used by most of the industry. It records snapshots of a project and lets many people work on it at once.

HTTP status code

A three-digit result for a request. 200 succeeded, 301 moved permanently, 404 not found, 429 too many requests, 500 server error.

Hydration

The step where a framework attaches interactivity to server-rendered HTML in the browser. Heavy hydration is a common cause of pages that look ready but do not respond.

Idempotency

A property where repeating an operation produces the same result as doing it once. Essential for safely retrying payment and order requests.

JSON

A plain-text format for structured data, built from objects, arrays, strings, numbers, booleans and null. The default format for web APIs.

Lazy loading

Deferring the download of something until it is needed — typically images below the fold or code for a feature the visitor has not opened.

Latency

The delay before a transfer begins. Distance, not bandwidth, dominates it, which is why CDNs exist.

Middleware

Code that runs between a request arriving and the handler that answers it — used for authentication, logging, compression and rate limits.

Minification

Removing whitespace and shortening names in code to reduce file size, without changing behaviour.

MVP

Minimum viable product: the smallest version that delivers real value, built to test whether the idea works before investing in the full scope.

ORM

Object-relational mapper. A library that lets you work with database rows as objects in your language instead of writing SQL by hand.

Pull request

A proposal to merge one branch into another, with a place for review and discussion before it lands.

Rate limit

A cap on how many requests a client may make in a period. Exceeding it usually returns HTTP 429.

Refactoring

Restructuring code without changing what it does, to make it easier to read, test or extend.

Regression

A bug that breaks something which previously worked. Automated tests exist mainly to catch these.

Repository

A project tracked by version control, including its full history.

REST

A common style for HTTP APIs: resources at predictable URLs, manipulated with the standard HTTP methods.

Responsive design

Building one layout that adapts to any screen size, rather than maintaining a separate mobile site.

Runtime

The environment that executes your code — a browser, Node.js, or a Python interpreter, for example.

Semantic HTML

Using elements for what they mean rather than how they look, so browsers, screen readers and crawlers can understand the structure.

Serverless

Running code in short-lived functions managed by a provider. You still use servers; you just do not operate them.

SPA

Single-page application. The browser loads one shell and JavaScript swaps the content, rather than requesting a new document per view.

SQL

The query language of relational databases, used to read, insert, update and delete rows.

SSG

Static site generation: pages are built once at deploy time and served as plain files, which is fast and cheap to host.

SSR

Server-side rendering: HTML is produced per request on the server, which helps first paint and crawlers.

Technical debt

The future cost of a shortcut taken today. Sometimes a sensible trade; always worth tracking deliberately.

Unit test

An automated check of one small piece of code in isolation, run on every change.

Webhook

A reverse API call: instead of you polling a service, it posts to a URL you supply when an event happens.

WCAG

The Web Content Accessibility Guidelines — the standard most accessibility requirements and laws are based on.

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