Application programming interface

An application programming interface (API) is a set of functions, procedures, methods or classes used by computer programs to request services from the operating system, software libraries or any other service providers running on the computer.^[1] A computer programmer uses the API to make computer applications.

An API works by making a structure for computers to interact with each other, including rules for interactions and methods the computers can use to quickly send data to each other.^[2] Because an API's goal is to connect computers with other computers, they are different from a user interface, which has the goal of connecting human users with computers.

Main types of web APIs:^[3]

Open APIs - available to the public; they can be accessed by any external users.
Partner APIs - available to strategic business partners; they are exposed to a public API developer portal.
Internal APIs - available to a company's internal development teams; they are exposed to a private API developer portal.
Composite APIs - are a sequence of tasks bundled into a single API call.

Types of API include web services API like the Twitter API, which allows programs to use the API to receive updates on tweets.

Application Programming Interface Media

Metaphorically, APIs connect software like interlocking blocks.
A diagram from 1978 proposing the expansion of the idea of the API to become a general programming interface, beyond application programs alone
Although the people who coined the term API were implementing software on a Univac 1108, the goal of their API was to make hardware independent programs possible.
Screenshot of Web API documentation written by NASA

References

↑ What is an API (Application Programming Interface)? (in en). Data Basecamp (2022-07-23). Retrieved 2022-08-11.
↑ Chen, Michael. What Is an API (Application Programming Interface)? (in en). Oracle (2025-02-24). Retrieved 2026-02-26.
↑ Defranchi, Lydia. Different types of APIs explained: styles, protocols, audiences + real-life examples (in en). Axway Software (2024-03-28). Retrieved 2020-04-30.

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[1] What is an API (Application Programming Interface)? (in en). Data Basecamp (2022-07-23). Retrieved 2022-08-11.

[2] Chen, Michael. What Is an API (Application Programming Interface)? (in en). Oracle (2025-02-24). Retrieved 2026-02-26.

[3] Defranchi, Lydia. Different types of APIs explained: styles, protocols, audiences + real-life examples (in en). Axway Software (2024-03-28). Retrieved 2020-04-30.

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