Application Programming Interface (API)
An application programming interface (API) is a set of tools and/or protocols that allow a programmer to build software applications. Most APIs have pre-determined inputs, outputs, operations, or functionality that are independent of their implementation.
You can think of APIs as building blocks that existing software offers to the public. Programmers can use these building blocks to make new things out of existing software.
You most likely use APIs every day without even knowing it. For example, most social networks have a 'Share' API that allows other apps to share their content to services like Facebook, Twitter, etc.