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.
      [1913 Webster]

   2. To illuminate with knowledge; to enlighten. [In this sense
      less common than enlighten.]
      [1913 Webster]

            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.
      [1913 Webster]

            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.
   [1913 Webster]

         O Lord, let thy mercy lighten upon us.   --Book of
                                                  Common Prayer
                                                  [Eng. Ed.].
   [1913 Webster]

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.
      [1913 Webster]

            This dreadful night,
            That thunders, lightens, opens graves, and roars
            As doth the lion.                     --Shak.
      [1913 Webster]

   2. To grow lighter; to become less dark or lowering; to
      brighten; to clear, as the sky.
      [1913 Webster]

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.
      [1913 Webster]

   2. To make less burdensome or afflictive; to alleviate; as,
      to lighten the cares of life or the burden of grief.
      [1913 Webster]

   3. To cheer; to exhilarate.
      [1913 Webster]

            Lightens my humor with his merry jests. --Shak.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

98 Moby Thesaurus words for "lighten":
   abate, allay, alleviate, assuage, attemper, attenuate,
   bank the fire, be light, beacon, blunt, break, brighten,
   brighten up, chasten, cheer up, constrain, control, damp, dampen,
   dawn, de-emphasize, deaden, dilute, diminish, disburden,
   disembarrass, disencumber, downplay, dull, ease, enlighten,
   extenuate, flood with light, floodlight, gladden, grow bright,
   grow light, have little weight, highlight, illume, illuminate,
   illumine, irradiate, keep within bounds, kick the beam, lay,
   lenify, lessen, light, light up, luminate, make light,
   make lighter, mitigate, moderate, modulate, mollify, obtund,
   off-load, overshine, palliate, play down, reduce,
   reduce the temperature, reduce weight, relieve, relume, relumine,
   restrain, set at ease, shed light upon, shine, shine upon, slacken,
   slow down, smile, smother, sober, sober down, soften, spotlight,
   stifle, subdue, suppress, tame, temper, thin, throw light upon,
   tone down, tune down, unballast, unburden, underplay, unfreight,
   unlade, unload, weaken, weigh lightly