API: Application Programming Interface(-description)
An API is a collection of defined interfaces for the activation of an instrument, an operating system or an application.
It is provided by the producer or another site to make the software creation for a system easier. Frequently used functions are provided as callable functions to standardise and simplify the accesses to parts of the system. To the developer the existence of an API means that he only must have the exact specifications and options of the respective software-library instead of special knowledge of the system.