A CD key is a specific software-based key for a certain program or a computer game. It is used to identify that the copy of the program/game is original. For official products the key might be used offline, at other times with software like Windows XP it is registered online to prevent multiple people using the same key.

The term CD key very likely comes from the fact that the key is usually on the case or the CD. Computer games use CD keys to verify that the game has been not been illegally copied; for example, one could not play Diablo II online without an original/unique CD key.

CD keys consist of a series of numbers and/or letters. This sequence is then passed to a verification function in the program. This function manipulates the key sequence according to a mathematical algorithm and attempts to match the results to a set of valid solutions.