Breath (WoW)  

When a player dives beneath the surface of water, they cannot stay there forever. A meter will appear indicating that player's breath, and when it empties, the player will start drowning and rapidly taking damage. Returning to the surface or finding another source of air will refill the meter, and it will disappear when it is full.

Undead characters have extra breathing capacity, and some items and class abilities remove the need to breath entirely.