A mixture of self-control, self-possession, and the ability to maintain composure in the face of attempted provocations. Composure is used to resist compulsions to act, while willpower is used to snap out of a state or act they are currently in. As a scale:
0: The type of lunatic that goes into a murderous rage due to a polite greeting.
1: Poor impulse control. Anger management issues. "What do you mean 'I'm funny'?"
2: Probably the average mix of some incidents they lost their composure and would prefer to forget coupled with moments they performed admirably under pressure.
3: Consistently difficult to rattle, handles high pressure situations easily.
4: A noble that might well die before losing face publicly, soldiers that calmly perform duties while under imminent threat of death.
5: While performing a speech, is attacked by an assassin, handles them and calmly finishes the speech.
6: Honey badgers care significantly more.