Testing carried out using no recognised test case design technique.
BS 7925-1.British Computer Society Specialist Interest Group in Software Testing (BCS SIGIST)

Closely linked to Error Guessing. However the phrase ad-hoc has connotations of being much more on whim of the tester.

Certainly in exploratory testing there is a need for ad-hoc measures. It is in exploratory testing that a tester can can get "a feel" for the application, without the confines of a strict testing regime.