The Collaborative International Dictionary of English v.0.48:

Equanimity \E`qua*nim"i*ty\, n. [L. aequanimitas, fr.
   aequanimus: cf. F. ['e]quanimit['e]. See Equanimous.]
   Evenness of mind; that calm temper or firmness of mind which
   is not easily elated or depressed; patience; calmness;
   composure; as, to bear misfortunes with equanimity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

equanimity
    n 1: steadiness of mind under stress; "he accepted their
         problems with composure and she with equanimity" [syn:
         composure, calm, calmness, equanimity] [ant:
         discomposure]