In computing, an application programming interface or API is a standardized set of classes, methods, functions and constants that serves as a front end through which one software program provides services to other software programs.

It is offered by a software library or web service, usually accompanied by a description that specifies how consumer programs can use the functionality of the provider program. There a different API architectures such as REST (representational state transfer), RPC, a remote procedural call protocol, or SOAP (simple object access protocol). These are web service APIs. Software libraries are also considered to be APIs. For instance, the fs module in JS is an API for the file system.