Shareware

Shareware is a software marketing concept where an incomplete or time-limited version of a program is released to entice people into buying the full (registered) version. Doom and Heretic were both initially released as shareware, each containing only the first respective nine-level episode in the unregistered version: Doom: Knee-Deep in the Dead and Heretic: City of the Damned.

Part of the reason Doom II was not given a shareware release, according to John Carmack, was that "[a] lot of people consider themselves to have "finished DOOM" when they just finished the shareware episode."