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]