Uld
Uldaman, an instance in Badlands
UC
Undercity, home of the Undead
Uber
German slang for 'super', originally meaning 'over'; exceptionally powerful
UB
The Underbog, a Coilfang Reservoir instance in Zangarmarsh
UA
Unstable affliction. A 41-point warlock talent in the affliction tree, a DoT that causes damage to anyone who dispels it.
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. Twinks usually have levels ending in 9 in order to dominate battlegrounds.
Trash
Enemies in an instance that are not bosses. In raids, trash will respawn after a certain amount of time if the associated boss is not killed.
TK
Tempest Keep, a group of instances in Netherstorm. Sometimes refers to The Eye, the Level 70 25-man raid wing of Tempest Keep.
Tier
Refers to a level of raiding content or class specific equipment. Tiers 1-3 are acquired in level 60 raid encounters, whereas T4-6 are in level 70 raids.
Threat
A value which determines who an NPC is attacking or focusing on. Once a player has aggro, other players in melee range need to reach 110% of that player's threat to take the aggro of the NPC. Players outside of melee ranged need to reach 130% to pull aggro.