Bind on Use
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.
Almost all mounts, companions and recipes are effectively bind-on-use: using one converts it from an item into an ability (an entry in a spellbook tab or profession window), and abilities cannot be traded.