Source : Webster's Revised Unabridged Dictionary (1913)

Lighten \Light"en\ (l[imac]t"'n), v. i. [See {Light} to alight.]
   To descend; to light.

         O Lord, let thy mercy lighten upon us.   --Book of
                                                  Common Prayer
                                                  [Eng. Ed.].

Lighten \Light"en\, v. t. [See {Light} not heavy.]
   1. To make lighter, or less heavy; to reduce in weight; to
      relieve of part of a load or burden; as, to lighten a ship
      by unloading; to lighten a load or burden.

   2. To make less burdensome or afflictive; to alleviate; as,
      to lighten the cares of life or the burden of grief.

   3. To cheer; to exhilarate.

            Lightens my humor with his merry jests. --Shak.

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.

Lighten \Light"en\, v. t. [See {Light} to illuminate.]
   1. To make light or clear; to light; to illuminate; as, to
      lighten an apartment with lamps or gas; to lighten the
      streets. [In this sense less common than light.]

            A key of fire ran all along the shore, And lightened
            all the river with a blaze.           --Dryden.

   2. To illuminate with knowledge; to enlighten. [In this sense
      less common than enlighten.]

            Lighten my spirit with one clear heavenly ray. --Sir
                                                  J. Davies.

   3. To emit or disclose in, or as in, lightning; to flash out,
      like lightning.

            His eye . . . lightens forth Controlling majesty.
                                                  --Shak.

   4. To free from trouble and fill with joy.

            They looked unto him, and were lightened. --Ps.
                                                  xxxiv. 5.

Source : WordNet®

lighten
     v 1: make more cheerful [syn: {buoy up}] [ant: {weigh down}]
     2: make more cheerful [syn: {buoy up}]
     3: make lighter or brighter; "The paint will brighten the room"
        [syn: {brighten}, {lighten up}] [ant: {darken}]
     4: become brighter; "The sky brightened" [syn: {brighten}]
        [ant: {darken}]
     5: become lighter; "The room lightened up" [syn: {lighten up}]