Ad hoc testing can, for example, be an extension of existing documented test cases but intend to apply invented variations of those test cases improvisationally without formally documenting the specifics beforehand.
However, as Desikan notes, to get the most from an ad hoc test and limit its downsides, the test should be properly documented post-execution and -completion, and the results report should address how any defects were identified in a step-by-step manner.