An Application Programming Interface (API) is a set of protocols, routines, and tools for building software applications. APIs define how software components should interact with each other, allowing different applications and systems to communicate with each other.

APIs provide a standardized way for developers to access the functionality of a particular software system or service, without having to understand the underlying code or infrastructure. For example, social media platforms like Twitter and Facebook provide APIs that allow developers to build applications that can access and interact with their services, such as posting tweets or retrieving user information.

APIs are typically defined by documentation that specifies how to use them, including the inputs and outputs of each function, the expected behavior of the API, and any authentication or security requirements. APIs can be public or private, depending on whether they are available for use by anyone or only by authorized users or applications.

By using APIs, developers can save time and effort by leveraging existing software components rather than building everything from scratch. APIs also promote interoperability between different systems and services, enabling greater connectivity and integration in the digital world.