The Collaborative International Dictionary of English v.0.48:

Lightness \Light"ness\, n. [From Light not heavy.]
   The state, condition, or quality, of being light or not
   heavy; buoyancy; levity; fickleness; nimbleness; delicacy;
   grace.
   [1913 Webster]

   Syn: Levity; volatility; instability; inconstancy;
        unsteadiness; giddiness; flightiness; airiness; gayety;
        liveliness; agility; nimbleness; sprightliness;
        briskness; swiftness; ease; facility.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lightness \Light"ness\, n. [From Light bright.]
   1. Illumination, or degree of illumination; as, the lightness
      of a room. --Chaucer.
      [1913 Webster]

   2. Absence of depth or of duskiness in color; as, the
      lightness of a tint; lightness of complexion.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lightness
    n 1: a feeling of joy and pride [syn: elation, high spirits,
         lightness]
    2: the property of being comparatively small in weight; "the
       lightness of balsa wood" [syn: lightness, weightlessness]
       [ant: heaviness, weightiness]
    3: the gracefulness of a person or animal that is quick and
       nimble [syn: agility, legerity, lightness,
       lightsomeness, nimbleness]
    4: having a light color [ant: darkness]
    5: the visual effect of illumination on objects or scenes as
       created in pictures; "he could paint the lightest light and
       the darkest dark" [syn: light, lightness]
    6: the trait of being lighthearted and frivolous [syn:
       lightsomeness, lightness]

Moby Thesaurus II by Grady Ward, 1.0:

208 Moby Thesaurus words for "lightness":
   Munsell chroma, achroma, achromasia, achromatosis, agility,
   airiness, airy texture, albescence, albinism, albino, albinoism,
   amiable weakness, anemia, ashiness, attenuation, big deal,
   blondness, bloodlessness, bounce, breakability, breeziness,
   brightness, brittleness, buoyancy, cadaverousness, canescence,
   carefreeness, chalkiness, chambering, changeableness, cheerfulness,
   chirpiness, chloranemia, chroma, chromatic color, chromaticity,
   collapse, color quality, colorimetric quality, cool color,
   coquettishness, creaminess, daintiness, deathly hue,
   deathly pallor, debonairness, delicacy, destructibility,
   diaphanousness, dilutedness, dilution, dimness, disintegration,
   dullness, easy virtue, effeminacy, effervescence, elasticity,
   emptiness, ethereality, exiguity, exility, expansiveness,
   exsanguination, faddishness, faddism, fadedness, faintness,
   fairness, featliness, fickleness, fineness, flightiness,
   flimsiness, flippancy, fluffiness, foolishness, fragility, frailty,
   frangibility, frivolity, frivolousness, frostiness, frothiness,
   futility, gaiety, gauziness, ghastliness, glaucescence,
   glaucousness, gossameriness, gracility, grizzliness, haggardness,
   hoariness, hue, human frailty, hypochromia, hypochromic anemia,
   idleness, inanity, inconstancy, indecisiveness, infirmity of will,
   inherent vice, insubstantiality, irresolution, jauntiness,
   laciness, lack of depth, lactescence, laxity, leukoderma, levity,
   light heart, lightheartedness, lightsomeness, liveliness,
   lividness, loose morals, looseness, lucence, lucency, lucidity,
   luminosity, luminousness, mercuriality, mercurialness, milkiness,
   mistiness, moral weakness, muddiness, neutral color, nimbleness,
   nugacity, paleness, pallidity, pallidness, pallor, paperiness,
   pastiness, pearliness, perkiness, pertness, prison pallor,
   promiscuity, purity, rarity, resilience, resiliency, sallowness,
   saturation, shallow-wittedness, shallowness, sickliness,
   sickly hue, silliness, silver, silveriness, skittishness,
   sleaziness, sleeping around, slenderness, slightness, slimness,
   snowiness, spryness, subtility, superficiality, swinging, tenuity,
   thinness, tint, tone, translucence, translucency, triflingness,
   triteness, triviality, trivialness, undependability,
   unpredictability, unprofoundness, unprofundity, unreliability,
   unsubstantiality, vacuity, vagueness, value, vanity, vapidity,
   velleity, vitiligo, vivacity, volatility, wanness, wantonness,
   warm color, wateriness, waywardness, weakness, white, white race,
   whiteness, whitishness, whorishness, wispiness, womanishness