An application-programming interface (API) is a set of programming instructions and standards for accessing a Web-based software application or Web tool.
API, an abbreviation of application program interface, is a set of routines, protocols, and tools for building software applications. The API specifies how software components should interact and are used when programming graphical user interface (GUI) components.
For example, Amazon.com released its API so that Web site developers could more easily access Amazon’s product information. Using the Amazon API, a third party Web site can post direct links to Amazon products with updated prices and an option to “buy now.”
API stands for Application Programming Interface, i.e. API is the way for an application to interact with certain system/application/library/etc. For example, there are API’s for OS (WinAPI), API’s for other applications (like databases) and for specific libraries (for example, image processing), etc. APIs are usually developed in a form consumable by a client application. For C/C++ applications, it a set header files and dynamic/static libraries. For Java – set of jars. And so on.