Depends on the context the words are used, and sometimes depending on the personell or the area involved.
Dielectric testing is also called Hi-Pot testing, using a test set that applies a very high voltage into a cable or device to be tested, and measures the milliamps that flow thru the insulation.
Another test is called Megger, which applies a much lower voltage into the cable or device, and measures the actual resistance in megohoms.
I have included a couple of articles from CE magazine (Compliance Engineering) that offers a detailed explanation of both methods:
http://www.ce-mag.com/archive/02/Spring/gray.html http://www.ce-mag.com/archive/01/Spring/Ahmadi.html
We use both methods, primarily Hi-Pot testing on Medium and High voltage cables (above 600 Volts), and Megger testing on low voltage cables (600 Volt and below) Hi-Pot testing can be destructive due to the high voltages used, and Megger is considered as Non-destructive.