The Collaborative International Dictionary of English v.0.48:

lightening \lightening\ n.
   The process of changing to a lighter color.

   Syn: whitening.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Lighten \Light"en\ (l[imac]t"'n), v. i. [imp. & p. p.
   Lightened (l[imac]t"'nd); p. pr. & vb. n. Lightening.]
   [OE. lightenen. See Light to kindle, illuminate.]
   1. To burst forth or dart, as lightning; to shine with, or
      like, lightning; to display a flash or flashes of
      lightning; to flash.
      [1913 Webster]

            This dreadful night,
            That thunders, lightens, opens graves, and roars
            As doth the lion.                     --Shak.
      [1913 Webster]

   2. To grow lighter; to become less dark or lowering; to
      brighten; to clear, as the sky.
      [1913 Webster]

WordNet (r) 3.0 (2006):

lightening
    n 1: descent of the uterus into the pelvic cavity that occurs
         late in pregnancy; the fetus is said to have dropped
    2: changing to a lighter color [syn: whitening, lightening]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "lightening":
   abatement, achromatization, allayment, alleviating, alleviation,
   alleviative, assuagement, blanching, bleach, bleaching, blunting,
   brightening, calming, damping, deadening, decoloration,
   decolorization, decolorizing, demulsion, diminution, disburdening,
   discoloration, disembarrassment, disencumbering, disencumberment,
   disencumbrance, dulcification, dulling, easement, easing,
   enlightening, etiolation, fading, falling-off, hushing,
   illuminating, illumining, leniency, lessening, letdown, letup,
   lighting, loosening, lulling, madder bleach, market bleach,
   mitigation, modulation, mollification, pacification, paling,
   palliation, quietening, quieting, reduction, relaxation, relief,
   relieving, remission, slackening, softening, soothing, subduement,
   tempering, tranquilization, unballasting, unburdening,
   unfreighting, unlading, unloading, unsaddling, untaxing,
   unweighting, whitening