Requires some form of alphanumeric key or similar to be presented to prove you own an authentic copy. A weak copy-protection mechanism, but more useful as GUID which it is also used for.
Alternate names: Product key, CD-key, Serial number, Unlock key, Registration key
The first video game about Serial key was released in 1998.
This can function as a form of DRM by preventing play as long as no correct key is provided, in some games this limitation only affects singleplayer or multiplayer but not both (commonly the latter is blocked if not both).