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An application programming interface (API) is a software-facing contract that lets one program request data or functionality another program makes available. It defines what a caller can ask for and how the provider responds; it does not have to be a website connection or a screen for people to use.
What does API stand for?
API stands for application programming interface. An API is an interface intended for software interaction: a program uses it to interact with another program or with capabilities exposed by its own environment. MDN describes APIs as features and rules that enable software interaction rather than interaction through a human user interface (MDN Web Docs).
The term “contract” is useful because an API sets expectations on both sides. It specifies the operations or data available and the rules a caller must follow, while keeping the provider’s unrelated internal implementation details out of the interaction (IBM).
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A client—the software making the request—uses the interface rules to ask a provider for an allowed operation or resource. The provider processes the request and returns a result. The API’s documentation describes how to form the request and what the caller can expect in response (AWS).
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- The client makes a request. It asks for a permitted function or piece of data in the format the API specifies.
- The provider handles it. The software offering the API checks and processes the request, including any applicable access controls.
- The provider returns a response. The client uses the result without needing to know how the provider implemented the requested capability.
For example, a weather app can request weather information from a weather service and display the returned data to its user. In this case, the app is the client, the service provides the information, and the API defines how they exchange it (AWS).
What are examples of APIs?
APIs are not limited to services that communicate across the internet. A browser can expose APIs that let page code use browser or device capabilities. For example, the Geolocation API can let code request location information available from a user’s device; the Web Animations API lets code animate page elements (MDN Web Docs).
A software library can also expose an API: other code calls its available functions according to its interface. These examples share the same basic idea—a defined way for software to use functionality—although they do not all work over a network (IBM).
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API is the broad term. A web API is used through web or network technologies. A REST API is a web API designed around REST architectural principles. In other words, REST is one style of API, not another name for all APIs (AWS).
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| Term | Meaning |
|---|---|
| API | A software interface defining available operations or data and the rules for using them. |
| Web API | An API used through web or network technologies. |
| REST API | A web API designed according to REST principles. |
Definitions can also be scoped to a particular standard. The Australian Government API Design Standard describes RESTful APIs in terms of resources identified by URIs and operations defined by HTTP methods; that is the standard’s definition of RESTful APIs, not a universal definition of every API (Australian Government API Design Standard).
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should you check when choosing or using an API?
Start with the contract rather than assuming that an API works like another one. Check:
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- Purpose and capabilities: what data or functions does it actually expose?
- Interaction style: does it run locally, in a browser, or over a network, and what architectural rules does it use?
- Request and response rules: what inputs are accepted, and what results or errors can the client receive?
- Access controls: who may call it, and what permissions or credentials are required? API rules and controls determine what the client is allowed to do (AWS AppSync documentation).
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