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.]
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            Signor Pietro, who had an admirable way both of
            composure [in music] and teaching.    --Evelyn.
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   2. Orderly adjustment; disposition. [Obs.]
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            Various composures and combinations of these
            corpuscles.                           --Woodward.
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   3. Frame; make; temperament. [Obs.]
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            His composure must be rare indeed
            Whom these things can not blemish.    --Shak.
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   4. A settled state; calmness; sedateness; tranquillity;
      repose. "We seek peace and composure." --Milton.
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            When the passions . . . are all silent, the mind
            enjoys its most perfect composure.    --I. Watts.
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   5. A combination; a union; a bond. [Obs.] --Shak.
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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]

Moby Thesaurus II by Grady Ward, 1.0:

100 Moby Thesaurus words for "composure":
   Buddha-like composure, Oriental calm, acceptance, aplomb, ataraxia,
   ataraxy, awelessness, calm disposition, calm of mind, calmness,
   comfort, constraint, contemplation, content, contentedness,
   contentment, control, cool, coolheadedness, coolness, countenance,
   discipline, ease, easy mind, entire satisfaction, euphoria,
   expectation, fulfillment, happiness, icy calm, imperturbability,
   imperturbation, independence, indisturbance, inexcitability,
   lucid stillness, marmoreal repose, mental composure, nirvana,
   nodding acceptance, nonamazedness, nonamazement, nonastonishment,
   nonmarveling, nonwonder, nonwondering, peace, peace of mind,
   peacefulness, philosophic composure, philosophicalness, philosophy,
   phlegm, placidity, placidness, possession, quiescence, quiescency,
   quiet, quiet mind, quietism, quietness, quietude, reconcilement,
   reconciliation, repose, resignation, rest, restfulness, restraint,
   sangfroid, satisfaction, satori, self-command, self-conquest,
   self-control, self-denial, self-discipline, self-government,
   self-mastery, self-possession, self-restraint, serenity, silence,
   silken repose, sleep, slumber, soothingness, stillness,
   tranquillity, unamazedness, unamazement, unastonishment,
   unimpressibleness, unruffledness, unsurprise, unsurprisedness,
   well-being, wise passiveness, wonderlessness