The gaming term "mob" is not to be confused with its usual meaning in the English language, i.e. a large disorderly crowd or an organized gang of criminals.
Mobs are also known as bad guys, beasts, enemies, monsters, non-player attackers, and opponents. The TERA-specific term for a boss mob is BAM (Big-Ass Monster) which was coined by players and used due to the sheer size of these opponents. These types of mobs also have a special trample feature, where just moving around will hurt players under their feet. The term "mob" is in widespread use in the MMORPG genre.
In TERA, mobs differ from NPCs in that they are usually monsters that share a common name with several others rather than having a unique name or title (but, see BAM mobs), are non-friendly to everyone, are non-interactive, and/or exist solely for the purpose of being killed. These are part of the main aspect of PvE, and are found throughout the game world. There are also named mobs that are either quest related which are usually stronger than the surrounding unnamed mobs or rare mobs that are often elite.
In TERA, mobs differ from critters in that critters are primarily for ambiance, provide no direct experience and no loot, and cannot be killed. There are subtle mechanical differences, such as critters cannot be targeted by use of the cursor (mobs can be).
The term was originally used in MUDs (Multi-User Dungeons/Dimensions/Domains), the text-based precursors to MMORPGs. In many MUDs, in-game objects are defined either as stationary item "objects" (ranging from fountains to wearable equipment that you could pick up) or "mobiles," shortened to "mobs" (this included any NPC, be they aggressive monster or friendly shopkeeper, even if they were not actually moving around due to STAY_ROOM or similar flags in their properties).