Mental Power

Mental Power is the measure of a Magic user's ability to perform more complex Spells. Essentially, this correlates to the strength of the mind.

Ability to perform more complex spells
Someone with extreme mental power could understand how to cast Universe Tier spells and cast them in the first place, wheres someone with low mental power will only be able to reasonably cast lower tier spells.

Ability to perform spells with higher intensity
The power of a spell is limited to how powerful the user's mind is, no matter how much energy they have stored in their Mental Reserve. A mentally powerful user can cast a Detect Lifesig to detect kilometers away, whereas a beginner may only be able to see a few meters away.

Training
Highly intelligent and learned people benefit from Mental Power, and a common way to increase mental power is to practice spells that are not too complex to fail casting completely, but complex enough to be a big challenge to cast - essentially behaving similarly to mental weightlifting.

In addition, there are several non-magical ways of increasing mental power, such as taking playing certain skilled video games to a masterful degree, or even learning something complicated like programming or engineering.

Mental Power is not specific to a specific field of knowledge - being a mentally powerful programmer does not necessarily mean not knowing how to cast non-programming spells. Instead, the ability to specialize in certain spells to reduce their mental load (i.e. lowering the "mana cost") is called Theoretical Knowledge.

Examples
The freaks with the highest mental power are Katie and The ChessMaster, who can complete very closely when it comes to the sheer complexity of spells they can cast. In fact, they were the ones who combined their power to perform the Great Reset, which is a spell that essentially involved recreating the whole universe from scratch, in such a way that an Alternate Earth with a mostly similar history to Original Earth was created.

However, there are a few freaks who can reasonably compete with such mental power, such all members of The Final Council, Dianna, Whimpers, Arky and Vanity.

See Magic Stats for more stats that are inherent to a magic user's mind.