Equanimity /ekwəˈnimədē / noun 

Definition:  1. The quality of being calm, even-tempered, and not easily irritated; composure 2. Steadiness of mind under stress or pressure

Etymology: Equanimity is derived from the Latin word “aequus” meaning “equal” or “leve,l” and “animus” meaning “mind.” It literally means to have an equal mind. It is a state of being calm and even-tempered, especially in difficult situations. The idea is that one will face things with acceptance rather than worry or dread. The term has been used since the early 1800s.

In a Sentence

I admire her valor and equanimity in the face of conflict with humans or other animals.

His parents could not believe his serene, almost supernatural sense of equanimity.

She was very nervous going into her surgery, but her equanimity during the procedure was encouraging.

Synonyms 

Poise, Composure, Serenity, Self-possession, Self-control

Antonyms 

Indifference, Lack of Care