Ad hoc testing
Also called: Informal testing
Informal testing with no plan or documentation, trying things to see what breaks.
Definition
Ad hoc testing is unplanned and unstructured: the tester tries things based on intuition and experience, without a charter or notes. It can find bugs quickly but is hard to repeat or measure, which is the main difference from exploratory testing. (Monkey testing, feeding random input, often with a tool, is a related idea.)
In practice
While waiting for a build, you randomly tap buttons on the settings screen very fast and discover that two dialogs open on top of each other.
Learn it in context
Lessons that use this term.