FairPlay - technical definition

A digital rights management (DRM) system from Apple that is used to encode copy protected music titles purchased at the iTunes online music store. Integrated with the AAC file format, FairPlay allows titles to be played in any number of iPods, but a restricted number of computers. The title can be converted to a regular audio CD track without limit, but if used in a playlist, there is a limit.

The FairPlay DRM works by generating random encryption keys for each title purchased and storing the keys in the user's computer and iPod. The keys are used to decrypt the AAC file before playing. In order to manage the restrictions, users are required to authorize and deauthorize every computer they want to play the titles on. See AAC and iTunes.