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:

decreased \decreased\ adj.
   made less in size or amount or degree. Opposite of
   increased. [Narrower terms: attenuate, attenuated, faded,
   weakened; belittled, diminished, small; cut, cut-rate;
   diminished, lessened; minimized; remittent;
   attenuated]

   Syn: reduced.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

decreased
    adj 1: made less in size or amount or degree [syn: decreased,
           reduced] [ant: increased]