The Collaborative International Dictionary of English v.0.48:

Composure \Com*po"sure\, n. [From Compose.]
   1. The act of composing, or that which is composed; a
      composition. [Obs.]
      [1913 Webster]

            Signor Pietro, who had an admirable way both of
            composure [in music] and teaching.    --Evelyn.
      [1913 Webster]

   2. Orderly adjustment; disposition. [Obs.]
      [1913 Webster]

            Various composures and combinations of these
            corpuscles.                           --Woodward.
      [1913 Webster]

   3. Frame; make; temperament. [Obs.]
      [1913 Webster]

            His composure must be rare indeed
            Whom these things can not blemish.    --Shak.
      [1913 Webster]

   4. A settled state; calmness; sedateness; tranquillity;
      repose. "We seek peace and composure." --Milton.
      [1913 Webster]

            When the passions . . . are all silent, the mind
            enjoys its most perfect composure.    --I. Watts.
      [1913 Webster]

   5. A combination; a union; a bond. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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