Boss

In video games in general, a boss is a particularly large or difficult computer-controlled character that must be defeated at the end of a segment of a game, whether it be for a level, an episode, or the very end of the game itself.

Doom/Ultimate Doom
The bosses of the Ultimate Doom episodes are:

Doom II/Final Doom
Doom II and the games based on it all have the same boss:

Special effects
In most of the levels above, defeating the boss monster(s) causes the level to exit. But instead, in E1M8, killing all the Barons triggers the special effect of lowering all walls with tag 666.

The same function (A_Boss_Death in p_enemy.c) is used in MAP07 of Doom II-based games to lower floors with tag 666 when all mancubuses have been killed, and to raise floors with tag 667 when all arachnotrons have been killed. Similarly in MAP32 (but using a different function), killing all the Commander Keens causes doors with tag 666 to open. Thus, these monsters can be considered bosses in these levels for which they play a special role.

Heretic
Bosses in Heretic: Shadow of the Serpent Riders: