An Application Programming Interface (API) is a set of rules and protocols that allow different software applications to communicate with each other. APIs define the methods and data formats that programs use to request services from operating systems, libraries, or other applications. APIs are essential in enabling interoperability between different systems, allowing developers to integrate third-party functionalities, such as payment processing, social media sharing, or geolocation services, into their applications. They can be public, private, or restricted to specific users. Modern APIs often use REST (Representational State Transfer) or GraphQL for web services, making them a critical component of web development.