API glossary definition

An API, or Application Programming Interface, lets software talk to other software. A developer writes a request in one program, and another program responds with the data or action the developer needs. You use APIs when an app needs weather, payments, maps, or user accounts without rebuilding everything from scratch.

Think of an API as a set of rules that tells you what to send and what you can expect back. It defines endpoints, request formats, authentication methods, and response structures. For example, a maps API might accept a location and return coordinates or route details. A payments API might accept an order amount and return a transaction status.

APIs typically run over HTTP, so you send a request through standard web methods like GET or POST. GET usually fetches data, and POST usually sends data to create or update something. The system returns a response with a status code such as 200 for success or 401 when authentication fails, plus a body that carries the result in JSON or XML.

APIs matter because they turn complex integrations into repeatable workflows. They let teams build features independently, as long as they follow the same interface contract. A front-end app can call an API for product data, and a back-end service can change its internal logic without breaking the front-end, as long as it keeps the same API behavior.

You will also see several API styles. REST APIs use resources and clear URL endpoints. GraphQL APIs let you ask for exactly the fields you want. SOAP APIs use strict XML messages and a formal contract. Even when the style changes, the goal stays the same, you connect one system to another with a defined interface.

If you plan integrations, you should read the API documentation closely. It tells you the required parameters, rate limits, error formats, and sample requests. That's the practical definition of an API, a contract for software communication.

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