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An '''Application Programming Interface''' ('''API''') is a connection between computers[[computer]]s or between [[computer programsprogram]]s. It is a type of software [[Interface (computing)|interface]], offering a service to other pieces of [[software]].<ref name=":0">{{cite book |last=Reddy |first=Martin |author-link= |date=2011 |title=API Design for C++ |url=https://books.google.com/books?id=IY29LylT85wC |publisher=Elsevier Science |page=1 |isbn=9780123850041}}</ref> A document or standard that describes how to build such a connection or interface is called an ''API specification''. A computer system that meets this standard is said to ''implement'' or ''expose'' an API. The term API may refer either to the specification or to the implementation.
In contrast to a [[user interface]], which connects a computer to a person, an application programming interface connects computers or pieces of software to each other. It is not intended to be used directly by a person (the [[end user]]) other than a [[computer programmer]]<ref name=":0">{{citebook |last=Reddy |first=Martin |author-link= |url=https://books.google.com/books?id=IY29LylT85wC |title=API Design for C++ |date=2011 |publisher=Elsevier Science |isbn=9780123850041 |page=1}}</ref> who is incorporating it into software. An API is often made up of different parts which act as tools or services that are available to the programmer. A program or a programmer that uses one of these parts is said to ''call'' that portion of the API. The calls that make up the API are also known as subroutines[[subroutine]]s, methods, requests, or endpoints[[communication endpoint|endpoint]]s. An API specification ''defines'' these calls, meaning that it explains how to use or implement them.
One purpose of APIs is to [[information hiding|hide the internal details]] of how a system works, exposing only those parts a programmer will find useful and keeping them consistent even if the internal details later change. An API may be custom-built for a particular pair of systems, or it may be a shared standard allowing [[interoperability]] among many systems.
The term API is often used to refer to web APIs, which allow communication between computers that are joined by the internet. There are also APIs for programming languages, software libraries, computer operating systems, and computer hardware. APIs originated in the 1940s, though the term did not emerge until the 1960s and 70s.▼
▲The term API is often used to refer to [[web APIsAPI]]s,<ref name="Lane2019"/> which allow communication between computers that are joined by the [[internet]]. There are also APIs for [[programming languageslanguage]]s, [[library (computing)|software libraries]], computer [[operating systemssystem]]s, and [[computer hardware]]. APIs originated in the 1940s, though the term did not emerge until the 1960s and 70s.
