Mental Power

Also referred to as mental energy, will power, resolve, etc. Mental power is a property that describes how much one's mind can can exert its will. All intelligent creatures have some level of mental power, and the more a creature is capable of using its mind the more mental power it usually has. 
Mental power is not static, in fact it fluctuates constantly, many different factors play into it most of which relate to the state of the being's mind with things like fear or sadness weaking mental power, and things like focus and encouragement increasing it. There is no known limit to mental power, but improving one's base line mental power takes active practice often requiring hardship to strengthen one's will to see any large improvement.
Although the uses of mental power are numerous, the most well known of its uses are supernatural abilities. Magic for instance drains a mage of their mental power every time they cast a spell, often novice mages complain of feeling foggy headed or depressed when first learning how to cast spells.