Equanimity
/ˌek.wəˈnɪm.ɪ.ti/ noun hardA state of mental calmness and composure, especially under pressure or stress.
She accepted the surprising diagnosis with a quiet equanimity that amazed everyone in the room.
Memory Trick
'Equ-' means even (as in 'equal'), and 'anim' means mind or spirit (as in 'animate') — equanimity is keeping an even mind no matter what happens around you.