Bind on Use
From WoWWiki
An item is Bind on Use (BoU) if it becomes Soulbound when first used. This is used for objects that are used but do not need to be equipped). A warning appears before a player uses a Bind on Use item for the first time, asking if the player is sure they want to do so.
Small pets, and other items can be BoU.
Other loot may be called Bind on Equip (BoE) or Bind on Pickup (BoP).
