An API (Application Programming Interface) is a set of rules and protocols that specifies how two software systems should interact with each other. It defines the way in which one system can access the functionality or data of another system, and allows developers to build applications that can communicate with other software or services.

APIs are commonly used to allow different software systems to work together, or to allow developers to access the functionality of a particular service or platform. For example, a social media platform might provide an API that allows developers to access user data or post updates on behalf of a user.

APIs are often designed as a set of functions or methods that can be called by other software, and they usually include detailed documentation that explains how to use them. APIs can be used to expose data or functionality over the internet, so that they can be accessed by remote systems or devices. They are an important part of modern software development and are used in a wide range of applications and industries.