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:

(MAP30: Last Call can be completed without killing the boss).

Doom 64
Bosses in Doom 64:

Heretic
Bosses in Heretic: Shadow of the Serpent Riders:

Hexen
Bosses in Hexen: Beyond Heretic:

Strife
Bosses in Strife: Quest for The Sigil:

Special characteristics
Except for immunity to the splash damage, Doom bosses generally have no special traits or abilities distinguishing them from other monsters.

Heretic bosses have several special features. They cannot be morphed by the Morph Ovum, tossed by the Iron Lich's whirlwind attack, nor be telefragged by the powered-up Firemace and, except for the Iron lich, are immune to the splash damage. To conform their large size, they receive less damage from the powered-up Hellstaff's rain of fire. Iron Lich in particular also takes reduced damage from the powered-up Dragon Claw's ripper projectiles. Once D'Sparil is dismounted, if he is hit with a Phoenix Rod's projectile or a powered-up Hellstaff projectile, he has a 37.5% chance of instantly teleporting away instead of receiving damage.

In addition, a special flag (MF2_BOSS) prevents lesser monsters from engaging in monster infighting with them, if provoked.

Special effects
In most of the levels above, defeating the boss monster(s) causes the level to end. In E1M8: Phobos Anomaly (Doom), however, killing all the barons lowers all walls with tag 666. In E4M6: Against Thee Wickedly, killing the cyberdemon causes a metal grate (four columns with tag 666) to blaze open, providing access to the exit switch.

The same function (A_BossDeath in p_enemy.c) is used in MAP07 of Doom II-based games to lower floors with tag 666 when all mancubi 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 the levels for which they play a special role.

In Heretic, the death of the last standing boss monster on a level invokes the special function (A_BossDeath), which kills all remaining non-boss monsters (by calling the P_Massacre) and, similar to Doom, activates sectors with tag 666.