equanimity
\ ee-kwuh-NIM-i-tee, ek-wuh- \, noun;
1.mental or emotional stability or composure, especially under tension or strain; calmness; equilibrium.
Quotes:
He responded to all these inquiries with equanimity , but in truth he couldn't wait to get out of there…
-- Elizabeth Royte, "Out of the Woods," The New Yorker , May 14, 2001
At times Omar's equanimity seemed a force, a strength to emulate. If he wasn't worried or frightened, then everything must be all right…
-- Mary McGarry Morris, "Songs in Ordinary Time ," 1995
Origin:
Equanimity can be traced to the Latin aequanimitas , which is formed with aequus meaning "even, plain, equal" and animus meaning "mind, spirit, feelings." It entered English in the early 1600s.
Dictionary.com