Bosses is an all-encompassing word that is often used to describe any overpowered single enemies who must be killed to progress in game, and cannot be avoided, but most of the time Act Bosses are meant by Bosses. Unique Monsters and Super Uniques are also often included.
Boss Fight is usually set in a specific location, with limited space to kite them. Often it will be very hard, if not impossible, to use a Town Portal, so preparation is advised. In Diablo III, every Boss fight takes place in an isolated chamber, and once it has started, there is no escape: the combat will not end until either side prevails. No mercy, no escape. Death or glory. Even to start a Boss Fight, all players in a group must click the "Yes" button. However, Rift Guardian bosses are spawned in usual locations, and it is possible (nobody said "easy") to escape the battle with them.
In addition to Act Bosses, the following enemies are included into Boss category:
- Skeleton King
- The Defiler
- Diablo (in the first game)
- Quest monsters like Zhar the Mad, Warlord of Blood, Countess, Blood Raven or Summoner.
- Uber Bosses, including Infernal Machine and Pandemonium Event
- All Rift Guardians
- All Keywardens (Odeg, Xah'Rith, Nekarat and Sokahr)
- Skeleton King
- Queen Araneae
- Zoltun Kulle
- Siegebreaker Assault Beast
All Bosses are characterized by their massive amounts of Life, many unique abilities they do not hesitate to use in combat (some of them capable of one-shot killing a hero, but usually boss skills are predictable and can be avoided), as well as increased damage. Many bosses are also assisted by hordes of minions, and may summon more at will.
Overall, boss fights are focused on player skill rather than raw damage per second and damage control, so players need to put all their effort into fight (unless, of course, they are way too overpowered for that boss and difficulty).