The Collaborative International Dictionary of English v.0.48:

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.]
      [1913 Webster]

            A key of fire ran all along the shore,
            And lightened all the river with a blaze. --Dryden.
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   2. To illuminate with knowledge; to enlighten. [In this sense
      less common than enlighten.]
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            Lighten my spirit with one clear heavenly ray. --Sir
                                                  J. Davies.
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   3. To emit or disclose in, or as in, lightning; to flash out,
      like lightning.
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            His eye . . . lightens forth
            Controlling majesty.                  --Shak.
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   4. To free from trouble and fill with joy.
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            They looked unto him, and were lightened. --Ps.
                                                  xxxiv. 5.
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The Collaborative International Dictionary of English v.0.48:

Lighten \Light"en\ (l[imac]t"'n), v. i. [See Light to alight.]
   To descend; to light.
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         O Lord, let thy mercy lighten upon us.   --Book of
                                                  Common Prayer
                                                  [Eng. Ed.].
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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.
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            This dreadful night,
            That thunders, lightens, opens graves, and roars
            As doth the lion.                     --Shak.
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   2. To grow lighter; to become less dark or lowering; to
      brighten; to clear, as the sky.
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The Collaborative International Dictionary of English v.0.48:

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.
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   2. To make less burdensome or afflictive; to alleviate; as,
      to lighten the cares of life or the burden of grief.
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   3. To cheer; to exhilarate.
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            Lightens my humor with his merry jests. --Shak.
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WordNet (r) 3.0 (2006):

lighten
    v 1: make more cheerful; "the conversation lightened me up a
         bit" [syn: lighten, lighten up, buoy up] [ant: weigh
         down, weigh on]
    2: reduce the weight on; make lighter; "she lightened the load
       on the tired donkey"
    3: become more cheerful; "after a glass of wine, he lightened up
       a bit" [syn: lighten, lighten up, buoy up]
    4: make lighter or brighter; "The paint will brighten the room"
       [syn: brighten, lighten up, lighten] [ant: darken]
    5: become lighter; "The room lightened up" [syn: lighten,
       lighten up] [ant: darken]
    6: alleviate or remove (pressure or stress) or make less
       oppressive; "relieve the pressure and the stress"; "lighten
       the burden of caring for her elderly parents" [syn:
       relieve, lighten]