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]