You can find variations on the definition of API all over the Internet. An API is a description of a software component in terms of its inputs, outputs, and operations. The inputs, outputs, and operations comprise the interface to the component (i.e., the application programming interface). In this sense, APIs have been part of system documentation for decades. However, there’s one key aspect to APIs that goes beyond good documentation housekeeping: APIs define interfaces and service usually without regard to implementation details.