Which of the following describes an API?

An Application Programming Interface, commonly known as an API, is a set of rules and protocols for building and interacting with software applications. It defines the methods and data formats that applications can use to communicate with each other, allowing different software systems to work together seamlessly.

APIs are crucial in modern software development as they enable the integration of different services and functionalities, allowing developers to leverage existing services without needing to know the internal workings of those services. This promotes efficiency, as developers can build applications faster by using the capabilities of other applications or platforms.

The other options refer to various concepts and terms that are distinct from what an API represents. An advanced protection initiative, an internal security protocol, and an advanced performance indicator do not capture the essence of how applications interact through defined interfaces.