An API or Application Programming Interface is a method which allows a program to communicate with another program. This includes interfaces that allow web browsers or web servers to communicate with other programs. (See also Active-X and Plug-In)
An Application Programming Interface (API) is an interface implemented by a software program to enable its interaction with other software. It is similar to the way the user interface facilitates interaction between humans and computers. APIs are implemented by applications, libraries and operating systems to determine the vocabulary and calling conventions. The programmer should employ it to use their services. It may include specifications for routines, data structures, object classes, and protocols used to communicate between the consumer and implementer of the API.