Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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