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An '''Application Programming Interface''' ('''API''') is a connection between [[computer]]scomputers or between [[computer program]]sprograms. 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">{{cite book |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 [[subroutine]]ssubroutines, methods, requests, or [[communication endpoint|endpoint]]sendpoints. An API specification ''defines'' these calls, meaning that it explains how to use or implement them.
The term API is often used to refer to [[web API]]sAPIs,<ref name="Lane2019"/> which allow communication between computers that are joined by the [[internet]]. There are also APIs for [[programming language]]slanguages, [[library (computing)|software libraries]], computer [[operating system]]ssystems, and [[computer hardware]]. APIs originated in the 1940s, though the term did not emerge until the 1960s and 70s.▼
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 API]]s,<ref name="Lane2019"/> which allow communication between computers that are joined by the [[internet]]. There are also APIs for [[programming language]]s, [[library (computing)|software libraries]], computer [[operating system]]s, and [[computer hardware]]. APIs originated in the 1940s, though the term did not emerge until the 1960s and 70s.
