{{short description|Set of subroutine definitions, protocols, and tools for building software and applications}}
[[An]] '''application programming interface''' ('''API''') is an [[Interface (computing)|interface]] or [[communication protocol]] between a [[Client–server model|client an]]<ref>{{Cite book|title=|last=dmwjt|first=dmwtjmad|publisher=dagjmwtj|year=djgptw|isbn=bjtpm|___location=ekarabari|pages=ajtjatj}}</ref>[[Client–server model|dand a server]] intended to simplify the building of client-side software. It has been described as a “contract” between the client and the server, such that if the client makes a request in a specific format, it will always get a response in a specific format or initiate a defined action.<ref name="Braunstein2018">{{cite book|last=Braunstein|first=Mark L.|title=Health Informatics on FHIR: How HL7's New API is Transforming Healthcare|url=https://books.google.com/books?id=tJdmDwAAQBAJ&pg=PA9|date=26 July 2018|publisher=Springer|isbn=978-3-319-93414-3|pages=9|quote=Non-technical readers may not understand what an API is. In non-technical terms it can be understood as a 'contract' that says to software developers that if you send a request from a 'client' computer (e.g., a phone, tablet, notebook or desktop) to a 'server' (the computer where the information is stored) in the specified format you will always get a response in a specified format or initiate a defined action.}}</ref>
An API may be for a web-based system, [[operating system]], [[database system]], computer hardware, or [[Library (computing)|software library]].
