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:

lessening \lessening\ adj.
   becoming less in dimension or degree; as, the lessening
   distance between us.
   [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

lessening \lessening\ n.
   a change downward; a decrease; a reduction.

   Syn: decrease, drop-off.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

lessening
    n 1: a change downward; "there was a decrease in his temperature
         as the fever subsided"; "there was a sharp drop-off in
         sales" [syn: decrease, lessening, drop-off] [ant:
         increase]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "lessening":
   abatement, abating, abridgment, allaying, allayment, alleviating,
   alleviation, analgesia, anesthesia, anesthetizing, appeasement,
   assuagement, assuaging, attenuation, attrition, blunting, calming,
   chastening, contraction, contractive, curtailment, cushioning, cut,
   cutting, dampening, damping, deadening, declining, decrease,
   decreasing, decrement, decrescence, decrescendo, decrescent,
   deduction, deflation, deliquescent, demulsion, depletion,
   depreciation, depression, derogation, detraction, diminishing,
   diminishment, diminuendo, diminution, dip, disparagement,
   dulcification, dulling, dwindling, dying, dying off, ease,
   easement, easing, extenuation, extraction, fade-out, falling-off,
   hushing, impairment, languishing, languishment, leniency, letdown,
   letup, lightening, loosening, lowering, lulling, miniaturization,
   mitigating, mitigation, modulation, mollification, numbing,
   on the wane, pacification, palliation, quietening, quieting,
   reducing, reduction, reductive, relaxation, relaxing, relief,
   remedy, remission, retraction, retrenchment, sagging, salving,
   scaling down, shortening, shrinkage, simplicity, slackening,
   softening, soothing, subduement, subduing, subsiding, subtraction,
   tempering, tranquilization, truncation, waning, weakening