Question

What is a 10 letter word meaning evenness of mind or temper?

Answer 1

Evenness means an equality between two species, or to sets of variables in mathematics, so evenness of mind and temper would be a tranquility or peace between these two parts of your body.
A ten letter word with this meaning is Equanimity. 

Definition: mental calmness, composure, and evenness of temper, especially in a difficult situation.