"Boss" is the term used for any monster that is unique and particularly powerful. Bosses will often have high combat levels or special attacks and defences, requiring particular weapons and equipment or a team of players to defeat them. As an incentive to fight them, bosses may drop especially valuable or powerful items when they are killed, or just a greater quantity of items than usual.
Quests often include powerful and/or unique enemies, which can be described as "quest bosses". The Dominion Tower is home to a large number of quest bosses - a more complete list is found under Category:Quest Bosses.