The Collaborative International Dictionary of English v.0.48:

Decrement \Dec"re*ment\, n. [L. decrementum, fr. decrescere. See
   Decrease.]
   1. The state of becoming gradually less; decrease;
      diminution; waste; loss.
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            Twit me with the decrements of my pendants. --Ford.
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            Rocks, mountains, and the other elevations of the
            earth suffer a continual decrement.   --Woodward.
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   2. The quantity lost by gradual diminution or waste; --
      opposed to increment.
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   3. (Crystallog.) A name given by Ha["u]y to the successive
      diminution of the layers of molecules, applied to the
      faces of the primitive form, by which he supposed the
      secondary forms to be produced.
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   4. (Math.) The quantity by which a variable is diminished.
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   Equal decrement of life.
      (a) The decrease of life in a group of persons in which
          the assumed law of mortality is such that of a given
          large number of persons, all being now of the same
          age, an equal number shall die each consecutive year.
      (b) The decrease of life in a group of persons in which
          the assumed law of mortality is such that the ratio of
          those dying in a year to those living through the year
          is constant, being independent of the age of the
          persons.
          [1913 Webster]

WordNet (r) 3.0 (2006):

decrement
    n 1: the amount by which something decreases [syn: decrease,
         decrement] [ant: increase, increment]
    2: a process of becoming smaller or shorter [syn: decrease,
       decrement] [ant: growth, increase, increment]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "decrement":
   abatement, ablation, abridgment, alleviation, attenuation,
   attrition, consumption, contraction, corrosion, curtailment, cut,
   cutting, dampening, damping, decrease, decrescence, deduction,
   deflation, deliquescence, depletion, depreciation, depression,
   derogation, detraction, diminishment, diminution, dip,
   disparagement, dissipation, dissolution, drain, dying, dying off,
   erosion, evaporation, exhaustion, expenditure, extenuation,
   extraction, fade-out, impairment, impoverishment, languishment,
   leakage, lessening, letup, loss, lowering, miniaturization, minus,
   mitigation, reduction, relaxation, remission, retraction,
   retrenchment, sagging, scaling down, shortening, shrinkage,
   simplicity, subtraction, truncation, using, using up, wastage,
   waste, weakening, wear and tear, wearing, wearing away