The Collaborative International Dictionary of English v.0.48:

Levity \Lev"i*ty\ (l[e^]v"[i^]*t[y^]), n. [L. levitas, fr. levis
   light in weight; akin to levare to raise. See Lever, n.]
   1. The quality of weighing less than something else of equal
      bulk; relative lightness, especially as shown by rising
      through, or floating upon, a contiguous substance;
      buoyancy; -- opposed to gravity.
      [1913 Webster]

            He gave the form of levity to that which ascended;
            to that which descended, the form of gravity. --Sir.
                                                  W. Raleigh.
      [1913 Webster]

            This bubble by reason of its comparative levity to
            the fluidity that incloses it, would ascend to the
            top.                                  --Bentley.
      [1913 Webster]

   2. Lack of gravity and earnestness in deportment or
      character; trifling gayety; frivolity; sportiveness;
      vanity. " A spirit of levity and libertinism."
      --Atterbury.
      [1913 Webster]

            He never employed his omnipotence out of levity.
                                                  --Calamy.
      [1913 Webster]

   3. Lack of steadiness or constancy; disposition to change;
      fickleness; volatility.
      [1913 Webster]

            The levity that is fatigued and disgusted with
            everything of which it is in possession. --Burke.

   Syn: Inconstancy; thoughtlessness; unsteadiness;
        inconsideration; volatility; flightiness.

   Usage: Levity, Volatility, Flightiness. All these words
          relate to outward conduct. Levity springs from a
          lightness of mind which produces a disregard of the
          proprieties of time and place.Volatility is a degree
          of levity which causes the thoughts to fly from one
          object to another, without resting on any for a
          moment. Flightiness is volatility carried to an
          extreme which often betrays its subject into gross
          impropriety or weakness. Levity of deportment, of
          conduct, of remark; volatility of temper, of spirits;
          flightiness of mind or disposition.
          [1913 Webster]

WordNet (r) 3.0 (2006):

levity
    n 1: feeling an inappropriate lack of seriousness [ant:
         gravity, solemnity]
    2: a manner lacking seriousness