'API' originates from English, specifically as an initialism of the phrase 'Application Programming Interface', formed in the context of computer science and software engineering in the mid-20th century.

'Application Programming Interface' evolved from earlier uses of the words 'application', 'programming', and 'interface' in computing; as software systems and libraries developed (early operating systems and subroutine libraries), the term came into use and was later shortened to the initialism 'API' for convenience.

Initially, the term typically referred to low-level library or OS interfaces for calling routines; over time it broadened to include web APIs, remote service endpoints, and higher-level service contracts describing how systems interact.