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:

Decreasing \De*creas"ing\, a.
   Becoming less and less; diminishing. -- De*creas"ing*ly,
   adv.
   [1913 Webster]

   Decreasing series (Math.), a series in which each term is
      numerically smaller than the preceding term.
      [1913 Webster]

WordNet (r) 3.0 (2006):

decreasing
    adj 1: becoming less or smaller [ant: increasing]
    2: music [ant: increasing]