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]

Moby Thesaurus II by Grady Ward, 1.0:

232 Moby Thesaurus words for "decrease":
   abate, abatement, abbreviate, abbreviation, ablate, ablation,
   abrade, abridge, abstract, allay, alleviate, apportion, astriction,
   astringency, attrition, bate, be eaten away, bottleneck, calibrate,
   calibrated, cervix, circumscribe, circumscription, clip, close,
   coarct, coarctation, compact, compactedness, compaction, compress,
   compression, compressure, concentrate, concentration, condensation,
   condense, consolidate, consolidation, constrict, constriction,
   constringe, constringency, consume, consume away, consumption,
   contract, contraction, contracture, corrode, count, cramp, crumble,
   curtail, curtailment, cut, cut back, cut down, cutting, damp,
   dampen, de-escalate, de-escalation, decline, decrement, deduct,
   deflate, deliquesce, deplete, depletion, depreciate, depreciation,
   depress, derogate, derogation, detract, detraction, die away,
   differentiate, diminish, diminuendo, diminution, dip, disparage,
   disparagement, dissipate, dissipation, dive, divide, downgrade,
   drain, draw, draw in, draw together, dribble away, drop, drop off,
   dwindle, dwindling, ease, ease off, ease up, eat away, ebb, erode,
   erosion, evaporation, exhaustion, expenditure, extract, extraction,
   fall, fall away, fall off, falling off, file away, fix,
   gradational, grade, gradual, graduate, graduated, hierarchic,
   hourglass, hourglass figure, impair, impairment, impoverishment,
   increase, isthmus, knit, knitting, languish, leach, leakage,
   lessen, lessening, let up, lighten, lower, lowering, measure,
   melt away, mitigate, narrow, narrow place, narrowing, neck, number,
   parcel, pare, peter out, plummet, plunge, progressive, pucker,
   pucker up, puckering, purify, purse, pursing, quantify, quantize,
   rate, rebate, recede, reduce, reduction, refine, regular,
   remission, remove, retraction, retrench, retrenchment, roll back,
   rub away, run low, sag, scalar, scale down, shade off, shorten,
   shortening, shrink, shrinkage, shrinking, shrivel up, simplify,
   sink, slacken, slackening, solidification, solidify, squander,
   step down, strangle, stranglement, strangulate, strangulation,
   striction, stricture, subduct, subside, subsidence, subtract,
   systole, tail off, take away, take from, taper, taper off, thin,
   thin out, trim, truncation, tune down, using, using up, wane,
   wasp waist, wastage, waste, waste away, wear, wear away, wearing,
   wearing away, weed, withdraw, wrinkle, wrinkling