Dictionary:

nonchalance

  (nŏn'shə-läns')

Casual lack of concern: “The contemptuous nonchalance of her trailed hand irritated him” (Elizabeth Bowen).

noun

A stable, calm state of the emotions: aplomb, balance, collectedness, composure, coolness, equanimity, imperturbability, imperturbableness, poise, sang-froid, self-possession, unflappability. Slang cool. See calm/agitation, feelings.

Definition: calmness
Antonyms: anxiety, concern

Tutor's tip: This word was used in the 2006 Scripps National Spelling Bee finals.

The noun has one meaning:

Meaning #1: the trait of remaining calm and seeming not to care; a casual lack of concern
  Synonyms: unconcern, indifference