API =  Application Programming Interface.

When one program is used to communicate with another program.

or in other words, we can say 

API = Inter program communication

API stands for Application Programming Interface. It is a set of protocols, routines, and tools used for building software applications. APIs provide a way for different software components to interact with each other, allowing them to exchange data and services in a standardized way.

APIs are commonly used to integrate different software systems, such as connecting a website to a payment gateway, or allowing a mobile app to retrieve data from a database. By providing a standardized interface, APIs make it easier for developers to build software that works together, reducing the need for custom integration code.

APIs can be public or private, depending on whether they are available for anyone to use or only accessible to specific organizations or developers. Some popular public APIs include those provided by social media platforms like Twitter and Facebook, which allow developers to build apps that can access and interact with user data.