The Collaborative International Dictionary of English v.0.48:

Nonchalance \Non`cha`lance"\, n. [F. See Nonchalant.]
   Indifference; carelessness; coolness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

nonchalance
    n 1: the trait of remaining calm and seeming not to care; a
         casual lack of concern [syn: nonchalance, unconcern,
         indifference]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "nonchalance":
   accidia, acedia, aloofness, apathy, ataraxia, ataraxy,
   benumbedness, blah, blahs, carelessness, casualness, comatoseness,
   detachment, disinterest, dispassion, disregard, disregardfulness,
   dullness, easygoingness, heartlessness, hebetude, heedlessness,
   hopelessness, inappetence, inattention, incuriosity, indifference,
   indiscrimination, inexcitability, insouciance, lack of affect,
   lack of appetite, lackadaisicalness, languidness, lethargicalness,
   lethargy, listlessness, mindlessness, negligence, numbness,
   offhandedness, passiveness, passivity, phlegm, phlegmaticalness,
   phlegmaticness, plucklessness, pococurantism, recklessness,
   regardlessness, resignation, resignedness, sloth, sluggishness,
   sopor, soporifousness, spiritlessness, spunklessness, stupefaction,
   stupor, supineness, torpidity, torpidness, torpor, unanxiousness,
   unconcern, unmindfulness, unsolicitousness, withdrawnness