Given-When-Then or GWT or given/when/then is a semi-structured way to write down test cases. They can either be tested manually or automated as browser tests with Selenium (software).
It derives its name from the three clauses used, which start with the words GIVEN, WHEN and THEN.
The Given-When-Then was observed and codified (or "invented") by Dan North as part of behavior-driven development.
The method is also common in support-driven development and in artificial intelligence development, and is sometimes extended as given/when/then/unless (with exceptions).
References
Given-When-Then Wikipedia(Text) CC BY-SA