Mana

From the helpfile:

Mana Points

Mana or magic points are a direct reflectant of how much magic you can use
before becoming exhausted. Mana is extremely important and is only used by
spell-casters. Each time you 'cast' a spell you will drain some amount of your
mana pool, the exact amount varies from one spell to the next. Mana regenation
is determined both by your intellect and your wisdom. For more information
regarding intellect or wisdom, see 'help intellect' or 'help wisdom', 
respectively. Additionally, mana regen rates include a 52 point regeneration
per phase, plus the percent regenerated per phase.

Score Sheet Topic

Score Sheet Topic: Mana

This field represents the current and maximum mana you have. Mana is used 
mostly for casting spells but can also be used for certain other aspects such
as magical shields.

For more information, see: help mana