MP5

MP5 (also known as MP/5) is a shortened term for the item bonus "Restores X mana per 5 sec." or "Mana per 5 seconds". Although the number given is based on 5 second intervals, the effect triggers every 2 seconds. For example, an item that grants 10 mana per 5 sec. restores 4 mana every tick.

Cataclysm
With World of Warcraft: Cataclysm, this was removed from items and replaced with Spirit. All healers were given a Meditation-like ability.