The Collaborative International Dictionary of English v.0.48:

Brighten \Bright"en\, v. t. [imp. & p. p. Brightened; p. pr. &
   vb. n. Brightening.]

   Note: [From Bright, a.]
   1. To make bright or brighter; to make to shine; to increase
      the luster of; to give a brighter hue to.
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   2. To make illustrious, or more distinguished; to add luster
      or splendor to.
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            The present queen would brighten her character, if
            she would exert her authority to instill virtues
            into her people.                      --Swift.
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   3. To improve or relieve by dispelling gloom or removing that
      which obscures and darkens; to shed light upon; to make
      cheerful; as, to brighten one's prospects.
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            An ecstasy, which mothers only feel,
            Plays round my heart and brightens all my sorrow.
                                                  --Philips.
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   4. To make acute or witty; to enliven. --Johnson.
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The Collaborative International Dictionary of English v.0.48:

Brighten \Bright"en\, v. i. [AS. beorhtan.]
   To grow bright, or more bright; to become less dark or
   gloomy; to clear up; to become bright or cheerful.
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         And night shall brighten into day.       --N. Cotton.
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         And, all his prospects brightening to the last,
         His heaven commences ere world be past.  --Goldsmith.
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WordNet (r) 3.0 (2006):

brighten
    v 1: make lighter or brighter; "The paint will brighten the
         room" [syn: brighten, lighten up, lighten] [ant:
         darken]
    2: become clear; "The sky cleared after the storm" [syn: clear
       up, clear, light up, brighten] [ant: cloud,
       overcast]

Moby Thesaurus II by Grady Ward, 1.0:

51 Moby Thesaurus words for "brighten":
   animate, beacon, boost, break, brighten up, buoy up, burnish,
   cheer, cheer up, dawn, encourage, enlighten, enliven, exhilarate,
   flood with light, floodlight, gild, gild the lily, give a lift,
   gladden, grow bright, grow light, hearten, highlight, illume,
   illuminate, illumine, inspire, inspirit, invigorate, irradiate,
   light, light up, lighten, liven, luminate, overshine, perk up,
   pick up, polish, rejoice, rejoice the heart, relume, relumine,
   shed light upon, shine, shine upon, spotlight, sweeten,
   throw light upon, vitalize