mana
Mana is basically how much energy you have to do certain abilities from certain classes. For example, a mage uses Mana almost all the time to cast spells, while a barbarian would need it to rage as well as go berserk.
Mana can be regenerated through certain abilities (such as trance for mages, among many), as well as through the use of consumables such as alcohol, food.
There are even rumors of certain items in the game that will allow for increased mana regeneration.
see also, Heal