A trash mob (or simply trash) is any monster that is not a boss.
Trash mobs are usually easier to kill than bosses, although there are some famous trash pulls which are actually harder than the boss they precede (such as the second-to-last group prior to High Warlord Naj'entus). Players usually don't like trash mobs since they are viewed as a tedious and time-consuming preamble to boss fights. Nonetheless, trash mobs in instances usually give reputation and have some useful drops, frequently some crafting material which cannot be obtained otherwise.
Etymology[]
The term trash mob derives from the term yard trash which was coined in the late 1990s MMORPG Everquest. In that game, a low-level dungeon, Unrest, was composed of a house which contained several bosses, surrounded by a front, side, and back yard. In the yards, there were yard trash. Lower level players shouted for groups to kill yard trash for experience either before they were high-leveled enough to kill the bosses or when all the bosses were camped.
Random facts[]
During the 2007 Blizzcon Dungeon & Raid panel, Blizzard came up with the backronym "Tactically Random A.I. Spawn Heuristic" for TRASH.