application program interface (API)

This definition is part of our Essential Guide: Platform as a Service: Expert advice for selecting a PaaS vendor
Contributor(s): Thomas Li

An application program interface (API) is code that allows two software programs to communicate with each other.

The API defines the correct way for a developer to write a program that requests services from an operating system (OS) or other application. APIs are implemented by function calls composed of verbs and nouns.The required syntax is described in the documentation of the application being called.

Typically, APIs are released for third-party development as part of a software development kit (SDK) or as an open API published on the Internet. If the applications are written in different languages or have been written for different platforms, middleware can provide messaging services so the two applications can communicate with each other.

Business interest in APIs grew with Web 2.0 mashups and executive dashboards that pull data from two or more sources. Cloud computing has fueled even more interest in APIs, as companies experiment with ways to integrate a cloud provider's service with on-premises systems or other cloud services.

See also: cloud provider API

This was last updated in July 2014

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