Source : Webster's Revised Unabridged Dictionary (1913)

Decrease \De*crease"\, v. t.
   To cause to grow less; to diminish gradually; as,
   extravagance decreases one's means.

         That might decrease their present store. --Prior.

Decrease \De*crease"\, n. [OE. decrees, OF. decreis, fr.
   decreistre. See {Decrease}, v.]
   1. A becoming less; gradual diminution; decay; as, a decrease
      of revenue or of strength.

   2. The wane of the moon. --Bacon.

Decrease \De*crease"\, v. i. [imp. & p. p. {Decreased}; p. pr. &
   vb. n. {Decreasing}.] [OE. decrecen, fr. OF. decreistre, F.
   d['e]cro[^i]tre, or from the OF. noun (see {Decrease}, n.),
   fr. L. decrescere to grow less; de + crescere to grow. See
   {Crescent}, and cf. {Increase}.]
   To grow less, -- opposed to increase; to be diminished
   gradually, in size, degree, number, duration, etc., or in
   strength, quality, or excellence; as, they days decrease in
   length from June to December.

         He must increase, but I must decrease.   --John iii.
                                                  30.

   Syn: To {Decrease}, {Diminish}.

   Usage: Things usually decrease or fall off by degrees, and
          from within, or through some cause which is
          imperceptible; as, the flood decreases; the cold
          decreases; their affection has decreased. Things
          commonly diminish by an influence from without, or one
          which is apparent; as, the army was diminished by
          disease; his property is diminishing through
          extravagance; their affection has diminished since
          their separation their separation. The turn of
          thought, however, is often such that these words may
          be interchanged.

                The olive leaf, which certainly them told The
                flood decreased.                  --Drayton.

                Crete's ample fields diminish to our eye; Before
                the Boreal blasts the vessels fly. --Pope.

Source : WordNet®

decrease
     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: {lessening}, {drop-off}] [ant: {increase}]
     2: a process of becoming smaller or shorter [syn: {decrement}]
        [ant: {increase}, {increase}]
     3: the amount by which something decreases [syn: {decrement}]
        [ant: {increase}]
     4: the act of decreasing or reducing something [syn: {diminution},
         {reduction}, {step-down}] [ant: {increase}]
     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: {diminish},
           {lessen}, {fall}] [ant: {increase}]
     2: make smaller; "He decreased his staff" [syn: {lessen}, {minify}]
        [ant: {increase}]