The Collaborative International Dictionary of English v.0.48:

Lessen \Less"en\ (l[e^]s"'n), v. t. [imp. & p. p. Lessened
   (-'nd); p. pr. & vb. n. Lessening.] [From Less, a.]
   To make less; to reduce; to make smaller, or fewer; to
   diminish; to lower; to degrade; as, to lessen a kingdom, or a
   population; to lessen speed, rank, fortune.
   [1913 Webster]

         Charity . . . shall lessen his punishment. --Calamy.
   [1913 Webster]

         St. Paul chose to magnify his office when ill men
         conspired to lessen it.                  --Atterbury.

   Syn: To diminish; reduce; abate; decrease; lower; impair;
        weaken; degrade.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lessen \Less"en\, v. i.
   To become less; to shrink; to contract; to decrease; to be
   diminished; as, the apparent magnitude of objects lessens as
   we recede from them; his care, or his wealth, lessened.
   [1913 Webster]

         The objection lessens much, and comes to no more than
         this: there was one witness of no good reputation.
                                                  --Atterbury.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lessen
    v 1: decrease in size, extent, or range; "The amount of homework
         decreased towards the end of the semester"; "The cabin
         pressure fell dramatically"; "her weight fell to under a
         hundred pounds"; "his voice fell to a whisper" [syn:
         decrease, diminish, lessen, fall] [ant: increase]
    2: make smaller; "He decreased his staff" [syn: decrease,
       lessen, minify] [ant: increase]
    3: wear off or die down; "The pain subsided" [syn: subside,
       lessen]

Moby Thesaurus II by Grady Ward, 1.0:

161 Moby Thesaurus words for "lessen":
   abate, ablate, abrade, abridge, abstract, allay, alleviate,
   allow for, amputate, anesthetize, appease, assuage, attemper,
   bank the fire, bate, be eaten away, benumb, blunt, chasten, clip,
   close, color, compress, constrain, consume, consume away, control,
   corrode, crop, crumble, curtail, cushion, cut, cut back, cut down,
   damp, dampen, de-emphasize, deaden, deaden the pain, decline,
   decrease, deduct, deflate, deliquesce, depreciate, depress,
   derogate, detract, die away, diminish, disparage, dive, downgrade,
   downplay, drain, drop, drop off, dull, dwindle, ease, ease matters,
   eat away, ebb, erode, extenuate, extract, fall, fall away,
   fall off, file away, foment, give relief, gloss over, ignore,
   impair, keep within bounds, languish, lay, leach, lenify, let up,
   lighten, lower, lull, make allowance for, melt away, mince, minify,
   mitigate, moderate, modulate, mollify, numb, obtund, pad, palliate,
   pare, play down, plummet, plunge, poultice, pour balm into,
   pour oil on, purify, reduce, reduce the temperature, refine,
   relieve, remove, restrain, retrench, roll back, rub away, run low,
   sag, salve, scale down, shorten, shrink, simplify, sink, slacken,
   slake, slow down, slur over, smother, sober, sober down,
   soft-pedal, soften, soothe, step down, stifle, stupe, subduct,
   subdue, subside, subtract, suppress, tail off, take away,
   take from, tame, temper, thin, thin out, tone down, truncate,
   tune down, underplay, varnish, wane, waste, waste away, weaken,
   wear, wear away, weed, whitewash, withdraw