Testing carried out using no recognized test case design technique and it is a test executed without prior planning; especially if the expected test outcome is not predicted beforehand. An undocumented test.                 

                 One of the best uses of ad hoc testing is for discovery. Reading the requirements or specifications (if they exist) rarely gives you a good sense of how a program actually behaves. Even the user documentation may not capture the “look and feel” of a program. Ad hoc testing can find holes in your test strategy, and can expose relationships between subsystems that would otherwise not be apparent. In this way, it serves as a tool for checking the completeness of your testing. Missing cases can be found and added to your testing arsenal.

                 Finding new tests in this way can also be a sign that you should perform root cause analysis. Ask yourself or your test team, “What other tests of this class should we be running?” Defects found while doing ad hoc testing are often examples of entire classes of forgotten test cases.