Boss is a general gaming term used for a mob which is considerably stronger than others in a certain area. Taking down a boss is the epitome of PvE, as the main objective in most dungeons involves killing the boss of dungeon, and this task is, of course, not an easy task most of the time.
Compared to typical mobs, bosses have several key characteristics. Bosses have a unique name, a much larger appearance than normal mobs and have an entire part of the screen allotted for the mob's name and a conspicuous hit point bar, which aids the player in monitoring the hit points and movements of the boss much easier. Some bosses, particularly in Abyss mode dungeons and nests, have increased resistance or complete immunity to particular effects and debuffs.
Bosses can be categorized as a mini-boss or a normal boss, with mini-bosses having higher stats than normal mobs, but have a less complicated battle mechanic or set of skills. At least one boss can be seen in hostile instances, but as many as two copies of the boss may appear in selected maps (like Nameless Tyrant Tomb) when playing the Abyss difficulty of that instance.
In more difficult dungeons, bosses may have as many as 4 or more bars or life, indicated by the color of the boss's hit point indicator. A boss' behavior often depends on this, as some skills are often used when a boss hits a particular amount of bars of HP.