https://en.oxforddictionaries.com/definition/api

API

https://en.wikipedia.org/wiki/API_testing

API testing is a type of software testing that involves testing application programming interfaces (APIs) directly and as part of integration testing to determine if they meet expectations for functionality, reliability, performance, and security

Since APIs lack a GUI, API testing is performed at the message layer

https://www.soapui.org/learn/functional-testing/api-testing-101.html

API testing is one of the most challenging parts of the chain of software and QA testing because it works to assure that our digital lives run in an increasingly seamless and efficient manner.

https://smartbear.com/learn/api-testing/what-is-api-testing/

API testing is one of the most challenging parts of the whole chain of software testing and QA testing because it works to assure that our digital lives run in an increasingly seamless and efficient manner.

https://blog.udemy.com/api-testing/

API testing is in many respects like testing software at the user-interface level, only instead of testing by means of standard user inputs and outputs, you use software to send calls to the API, get output, and log the system’s response.

https://www.guru99.com/api-testing.html

It enables communication and data exchange between two separate software systems. A software system implementing an API contains functions/sub-routines which can be executed by another software system

Api Testing is entirely different from GUI Testing and mainly concentrates on the business logic layer of the software architecture