An application programming interface (API) is a set of rules and protocols that allows different software applications to communicate with each other. It defines the methods and data formats that applications can use to request and exchange information, making it easier for developers to integrate different services and functionalities into their own applications. APIs are commonly used in web development, enabling applications like social media platforms and payment gateways to share data and features. By using APIs, developers can save time and resources, as they can leverage existing services instead of building everything from scratch.