Source : Webster's Revised Unabridged Dictionary (1913)

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.

         And night shall brighten into day.       --N. Cotton.

         And, all his prospects brightening to the last, His
         heaven commences ere world be past.      --Goldsmith.

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.

   2. To make illustrious, or more distinguished; to add luster
      or splendor to.

            The present queen would brighten her character, if
            she would exert her authority to instill virtues
            into her people.                      --Swift.

   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.

            An ecstasy, which mothers only feel, Plays round my
            heart and brightens all my sorrow.    --Philips.

   4. To make acute or witty; to enliven. --Johnson.

Source : WordNet®

brighten
     v 1: make lighter or brighter; "The paint will brighten the room"
          [syn: {lighten up}, {lighten}] [ant: {darken}]
     2: become clear; "The sky cleared after the storm" [syn: {clear
        up}, {clear}, {light up}] [ant: {overcast}]
     3: become brighter; "The sky brightened around noon"; "Her face
        brightened when she heard the good news" [ant: {darken}]
     4: become brighter; "The sky brightened" [syn: {lighten}] [ant:
         {darken}]