How do you define an API?

An API (Application Programming Interface) is a set of operations a software component (i.e., a system, a sub-system, class, or a function) provides to its clients. For example, we use the API of the String class provided by C#/C++/Java library in our programming.

For More Information Please Refer: https://www.comp.nus.edu.sg/~damithch/guide/APIs.html