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]