The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         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.
          [1913 Webster]

                The olive leaf, which certainly them told
                The flood decreased.              --Drayton.
          [1913 Webster]

                Crete's ample fields diminish to our eye;
                Before the Boreal blasts the vessels fly.
                                                  --Pope.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Decrease \De*crease"\, v. t.
   To cause to grow less; to diminish gradually; as,
   extravagance decreases one's means.
   [1913 Webster]

         That might decrease their present store. --Prior.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. The wane of the moon. --Bacon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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: decrease, lessening, drop-off] [ant:
         increase]
    2: a process of becoming smaller or shorter [syn: decrease,
       decrement] [ant: growth, increase, increment]
    3: the amount by which something decreases [syn: decrease,
       decrement] [ant: increase, increment]
    4: the act of decreasing or reducing something [syn: decrease,
       diminution, reduction, step-down] [ant: increase,
       step-up]
    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]