Diablo 3

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AC - Armor Class or Defense.
Add - An extra monster has joined an existing battle.
Alt - A character on your account other than your main character.
AoE - Area of Effect Damage. This is a spell that hurts a group of monsters in an area like Blizzard in Warcraft III.
AE - Area Effect
AFK - Away from Keyboard. This means the player is away from his computer. 
Aggro - This means the monsters are mad at you and you’ve “activated” them to attack you. They are now in the motion of trying to reach and attack you. “The Murloc aggroed on me!” or “The Murlocs will aggro if you get too close!”
Aggro Radius - The radius around the monsters where they will “wake up” and attack you.
AGI - Agility
Avatar - Your character
BG - Battleground
Buff - A beneficial spell cast on a monster or player. An example of a “buff” is the Priest’s Inner Fire or Shaman’s Bloodlust.
Carebear - Player that prefers to help other players attack monsters rather than attack other players in player vs. player combat.
Caster - A character that stays on the back row to heal or cast spells on the enemy, such as a Mage.
Cheese - To exploit an imbalance in the game.
Combat Pets - A NPC controlled by a player, aiding that player and his teammates in fights.
CR - Corpse Retrevial
Creep - Monster
Creep Jacking - A term from Warcraft 3 where players attack other players while they are already engaged in combat with neutral monsters.
Critters - Monsters that don’t attack back, like a bunny or deer.
DD - Direct Damage. This is a spell that does all of its damage in one hit rather than spreading its damage over time. 
DMG - Damage
DOT - Damage over time
DPS - Damage per second
De-Buff - A negative spell cast on a unit that makes it less powerful. An example of a “de-buff” is Slow.
FH - Full Health
FM - Full Mana
FTL - For The Loss, a sports or game term.
FTW - For The Win, a sports or game term.
Gank - Player vs. Player: To attack another player while they’re trying to fight a monster.
GM - Game Master. Someone employed by Blizzard Entertainment to assist and help players.
Griefer - A person who purposely tries to annoy or anger other players.
Grinding - Staying in the same area fighting the same types of monsters for a very long time.
GS - Goldshire
Hate - Similar to threat
HP - Hit points or Health
Incoming (INC) - This means an attack is coming.
Kiting - A style of combat in which a player continually stays out of combat range of an enemy usually by running from it, while simultaneously causing damage to it.
KOS - Killed on Sight. If an Orc approaches a Human Guard, the Guard would try to kill on sight (aggro).
KS - Kill Steal. Try to steal another person’s kill.
LFG - Looking for a group.
LFM - Looking for more.
Log - When you log off; disconnect from the game.
LOL - Lots of Laugh or Laughing Out Loud. This abbreviation indicates something is really funny. It’s pretty cliche nowadays though.
LOM - Low on Mana.
LOS - Line of Sight
LOOT - To take the treasure from a monster that has been killed or from a chest.
LVL - Level
Mez - Short for Mesmerize. Refers to spells, such as Polymorph that temporarily incapacitate a target.
MMO - Massively Multiplayer Online.
MMOG - Massively Multiplayer Online Game
MMORPG - Massive Multiplayer Online Role Playing Game
MOB - MOB” is an old programming acronym of Mobile Object Block. Mobs are all computer controlled characters in the game. You should likely use some other term such as monster, creep, or bad guy.
MT - Main Tank
Named - A special monster that is usually stronger than surrounding monsters with possible special abilities and item drops.
Nerf - To downgrade, to be made softer, or make less effective. “X has been nerfed.”
Newbie - A term that sometimes means new player. Newbie is also used to suggest that a player is not very good.
Newb - Short for Newbie. See above. 
Ninja - To try to loot an item without other players knowing or paying attention. Basically, to take an item without permission.
N00b - An incredibly uncool way to say newb. Don’t use it. 
NPC - This is a non player-controlled character. The characters are controlled by the server or realm. A “computer” character. 
OOM - Out of Mana. People say this to let their party know they are out of mana and can’t cast any spells, especially healing.
Pat - Patrolling monster
PC - Player controlled character
Pet - A NPC controlled by a player such as a Wolf, Infernal, and so on. 
PK - Player Kill or Player Killer.
POP - Contraction of “Repopulation”, often used as a shortened term for the re-spawn of monsters. 
Proc - Activate. Example: A weapon with a special effect will “proc” every so often. 
PST - Please Send Tell. Indicates that the person speaking wants to receive communication via a /t(ell) or /w(hisper) command.
Puller - Person who pulls monsters for the party.
Pulling - One of the players in a party heads out and leads one or more of the monsters back to the party so that the party can attack the monsters. The idea is to prevent too many monsters from attacking at once.
PvE - Player vs. Environment. Combat between players and computer controlled opponents.
PvP - Player vs. Player. This means for one player to attack another player.
Res - Short for resurrect
Re-Spawn - A monster that has been killed has spawned (been created) again.
Rez - Resurrect
Root - To trap a target in place/stuck using a “root” type spell such as Entangling Roots.
Spawns - The location or process of monsters appearing when they are created in the world.
Stack - A number of identical items placed in a single inventory slot, to conserve space. Only certain items can be stacked. 
STR - Strength
Tank - A melee character that can take a lot of damage like a Warrior.
Tap - To do damage to a monster, making it “your” kill. Once you have damaged the monster, you are the only one able to get experience and loot from it. A monster with a greyed-out name bar has been tapped by another player and will not earn you experience or loot.
Taunt - Related to Aggro. An ability that allows a player to pull the attention of a monster off of another player and onto him- or herself.
Threat - Related to Aggro. This is what a character “gives off” to generate or draw monster aggro.
Train - To lead monsters into another player. This is not a desired behavior. 
Twink - A low level character who has been made more powerful by higher level characters, usually by getting stronger armor and weapons than the character would normally have at such a low level.
Uber - German slang for ‘super’, originally meaning ‘over’; exceptionally powerful
Vendor Trash - An item that only a vendor/merchant would buy
WoW - World of Warcraft
WTB - Wanting to buy
WTS - Wanting to sell
XP or Exp - Experience Points
Zerg - From StarCraft, to attack something with a lot of players/units.