Pair Testing is a type of Ad-hoc testing. When ever test engineers found the risk in functional and system testing like lack of time, lack of knowledge then they'll conduct the functional and system testing in Ad-hoc manner. That is coducting the testing in informal manner without follwing any formal testing type.

Pair means group together with testers and developeres. due to the lack of time developers and testers together to do the development and testing.