A Boss is an enemy that is significantly stronger and more dangerous than other Specimens. Their arrival is signaled with a cutscene and in Killing Floor 2 a healthbar will appear at the top of the HUD.
Killing Floor: Calamity
Killing Floor: Calamity functions the same as the first game, with the Patriarch being the only boss.
Killing Floor 2
Killing Floor 2 brought back the Patriarch, as well as introducing three more bosses; Dr. Hans Volter, King Fleshpound, and the Abomination. Which boss the players will face is random each game. The cutscene for each boss contains the name, the boss taunting the players, and a random tip to help the players defeat the boss.
There were also rumors of possibly adding a Matriarch boss, but those plans may have been scrapped.
Killing Floor: Incursion
Killing Floor: Incursion has the Scrake, the Fleshpound, and Husks upgraded to be boss enemies, along with a new boss in the Vile Matron. In Story Mode they are always fought only at the end of their respective levels. Weaker versions of all but the Vile Matron can appear during Holdout Mode, however.