Aggro - Aggression: the measure of a mob's attack interest in a specific character. "Holding Aggro" means the mob is fixed on you, so it's focusing its attacks on you. (Generally good for the Tank to Hold Aggro)

AoE - Area of Effect: dealing damage to enemies standing within a certain range radiating out from the spot where the spell/attack was targeted

Buff - Spell that increases a beneficial attribute to a player (can also be obtained through foods/potions/etc, depending on the game)

CC - Crowd Control: as an attack, it controls several enemies by either freezing, slowing, or generally inhibiting their attack abilities so your team can attack more efficiently

Cooldown - After certain attacks or spells are used, the cooldown is the time before the attack can be used again

Crit Hit - Critical Hit: When the maximum amount of damage for an attack is reached, it is called a critical hit

Debuff - Spell that decreases a beneficial attribute of a Mob or player (can also be obtained through poisons/etc, depending on the game)

DOT - Damage over Time: When a spell or attack deals damage over a period of time

DPS - Damage Per Second

EXP - Experience: points received for killing mobs, completing quests, etc; used to increase in level and strength

Grinding - Killing mobs for experience without regard to a quest or other goal

HP / Health Points / Hit Points - measure of a character's life; decreases upon injury

Instance - A specific area that exists only to a particular to a specific player or group of players

Kill Steal - Killing a Mob another player was in the process of attacking (generally to steal the EXP and/or loot drop)

Kite/Kiting - Tactical technique that reduces the amount of melee damage taken; generally involves using ranged attacks, then running away, then ranged attack, then running, all while the mob chases you until it dies (hopefully without ever getting near enough to land a hit)

Leech - Stealing experience and/or items; Sometimes a player will follow closely behind a group of a higher-level player as they perform a specific quest. The group does all the work clearing a path to the quest and the leech can then complete the quest without doing all the work

Loot - Objects obtained as rewards for killing mobs, completing quests, opening chests, etc.

Melee - Close-combat attacks performed with martial arts or with melee weapons: swords, axes, hammers, daggers, scythes, etc

MMORPG - Massively Multiplayer Online Role-Playing Game

Mob - "Mobile objects" / "Mobile Object Blocks" OR "Monster or Beast": generic term for a non-player entity whose primary purpose is to be killed for experience, quest objective, or loot. They tend to be aggressive to all players

MP / Mana Points / Magic Points - a measure of how much magic a character has to cast spells

MPK - Monster Player Kill: luring a monster to kill another player

Nerf - To reduce the capabilities, power, or attributes of a weapon or attack, or even an entire race/class; in an attempt by the developers to balance gameplay

Noob/n00b/newb (etc) - New or inexperienced player (generally an insult)

NPC - Non-Player Character (entity in-game controlled by the game/computer)

PC - Player Character (character controlled by a human player)

PK - Player Kill: killing another player

PvE - Player vs Environment: Fighting against Mobs and other game-controlled enemies

PvP - Player vs Player: Fighting against player-controlled characters

Regen - Regenerate: depending on the game, mobs and/or players may have functions to automatically regenerate HP, MP, and/or SP. Mobs, especially elite mobs, often have an HP regen function that allows them to regain HP during a fight, so the players fighting have to deal more damage per second than the mob can heal

SP / Stamina Points - measure of a character's stamina; meaning greatly changes depending upon the gameplay

Stun - Attack that stops a player or mob from attacking (and usually moving at all); usually only lasts a short time

Tank - Character with inherent high armor factor, high defense, high strength; they can take more damage than most characters. Used to Hold Aggro during team fights so lower-defense characters don't take damage. (Also makes it easier on healers if they only have to focus on Tanks)