MP5 (also known as MP/5) is a shortened term for the item bonus "Restores X mana per 5 sec." Although the number given is based on 5 second intervals, the effect will trigger every 2 seconds. For example, an item that grants 10 mana per 5 sec will restore 4 mana on every "tick". Mana regeneration depends on Spirit and MP5 bonuses, in most cases Spirit only effects regen outside of the 5 second rule while MP5 bonuses apply at all times.
See also: Formulas:Mana Regen