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

Moby Thesaurus II by Grady Ward, 1.0:

165 Moby Thesaurus words for "levity":
   absentmindedness, absurdity, airiness, amusement, ascent, banter,
   big deal, booing, bounce, breeziness, bubbliness, buoyancy,
   carefreeness, carelessness, catcalling, chaffing, changeableness,
   chirpiness, coquettishness, daintiness, debonairness, delicacy,
   derision, disregard, disregardfulness, distraction, downiness,
   emptiness, ethereality, facetiousness, faddishness, faddism,
   fickleness, fleering, flightiness, flimsiness, flippancy,
   floatability, fluffiness, foaminess, folly, fooling, foolishness,
   frivolity, frivolousness, frothiness, fun, futility, gentleness,
   giddiness, glee, gleefulness, gossameriness, grinning,
   heedlessness, high glee, hilariousness, hilarity, hissing, hooting,
   idleness, inadvertence, inadvertency, inanity, inattention,
   inattentiveness, inconsideration, inconstancy, incuriosity,
   indifference, inobservance, jauntiness, jeering, jocularity,
   jocundity, jolliness, jollity, joshing, joviality, joy, joyfulness,
   joyousness, lack of weight, laughter, leering, levitation,
   light heart, lightheartedness, lightmindedness, lightness,
   lightsomeness, mercuriality, mercurialness, merriment, merriness,
   mirth, mirthfulness, mockery, negligence, nonobservance, nugacity,
   obliviousness, panning, perkiness, pertness, playfulness,
   prankishness, pranksomeness, ragging, raillery, rallying, razzing,
   regardlessness, resilience, ridicule, roasting, roguishness,
   scoffing, shallowness, silliness, skittishness, slenderness,
   slightness, smart-aleckiness, smartness, smirking, sneering,
   snickering, sniggering, snorting, softness, sportiveness,
   superficiality, taunting, tenderness, thoughtlessness, trickery,
   trickiness, triflingness, triteness, triviality, trivialization,
   trivialness, twitting, unalertness, unawareness, unconsciousness,
   undependability, unheaviness, unheedfulness, unintentiveness,
   unmindfulness, unobservance, unpredictability, unreliability,
   unwariness, unwatchfulness, vacuity, vanity, vapidity, volatility,
   waggery, waggishness, weightlessness, yeastiness