Source : Webster's Revised Unabridged Dictionary (1913)

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.

         Charity . . . shall lessen his punishment. --Calamy.

         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.

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.

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

Source : WordNet®

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 fall to under a
          hundred pounds"; "his voice fell to a whisper" [syn: {decrease},
           {diminish}, {fall}] [ant: {increase}]
     2: make smaller; "He decreased his staff" [syn: {decrease}, {minify}]
        [ant: {increase}]
     3: wear off or die down; "The pain subsided" [syn: {subside}]