AB
Arathi Basin, a 15-man battleground with 5 bases whose objective is to capture and hold bases to collect victory points.
AC
Auchenai Crypts, an Auchindoun instance in Terokkar Forest
Add
An additional enemy who has joined in an active battle
AFK
Away From Keyboard
Aggro
Enter combat with or cause to enter combat. Most often caused by proximity or direct attack, but can be a scripted event. Also means the attention of an NPC in combat.
AGI
Agility, a base statistic
AH
Auction House
Alt
A alternate character, as opposed to a main character or primary character
AoE
Area of Effect – a spell or ability that affects anything within a given space. Most often refers to damage, but can be healing, buffing, or debuffing as well.
AQ
Ahn'Qiraj, a set of level 60 raid instances in Silithus
Arc
The Arcatraz, a Tempest Keep instance in Netherstorm
AV
Alterac Valley, a battleground with 40 players on each side. The main objective is to kill the opposition's general
Avatar
The representation of a player's character in the world
Badge
Badge of Justice, dropped from bosses in Heroic dungeons and Raids, exchanged for Epic level items
BC
The Burning Crusade. This is the first expansion to World of Warcraft which added the Outlands and raised the level cap to 70.
BF
Blood Furnace, a Hellfire Citadel instance in Hellfire Ramparts
BFD
Blackfathom Deeps, an instance in Ashenvale
BG
Battleground, an area dedicated to PvP encounters between alliance and horde
Blue
Rare level items, better quality than greens but worse than purples
BM
Black Morass, a Caverns of Time instance in Tanaris. Also, Beast Mastery, a Hunter talent tree.