Source : Webster's Revised Unabridged Dictionary (1913)

Lighten \Light"en\ (l[imac]t"'n), v. i. [imp. & p. p.
   {Lightened} (-'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.

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

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

Source : WordNet®

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}]