An application programming interface (API) is a set of rules that allows software programs to communicate with each other. It acts as an interface enabling interaction between different software systems. APIs are utilized when multiple systems need to collaborate to perform tasks they cannot achieve independently.

1. Application programming interface (API)
Solution
What is an API?
An application programming interface (API) is a particular set of rules ('code') and
specifications that software programs can follow to communicate with each other . It serves
as an interface between different software programs and facilitates their interaction, similar to the
way the user interface facilitates interaction between humans and computers.
How is it used?
According to the rules specified in the contract. The whole point of an API is to define how it's
used.
When and where is it used?
It's used when 2 or more separate systems need to work together to achieve something they
can't do alone.