The Collaborative International Dictionary of English v.0.48:

Diminish \Di*min"ish\, v. t. [imp. & p. p. Diminished; p. pr.
   & vb. n. Diminishing.] [Pref. di- (= L. dis-) + minish: cf.
   L. diminuere, F. diminuer, OE. diminuen. See Dis-, and
   Minish.]
   1. To make smaller in any manner; to reduce in bulk or
      amount; to lessen; -- opposed to augment or increase.
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            Not diminish, but rather increase, the debt.
                                                  --Barrow.
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   2. To lessen the authority or dignity of; to put down; to
      degrade; to abase; to weaken.
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            This doth nothing diminish their opinion. --Robynson
                                                  (More's
                                                  Utopia).
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            I will diminish them, that they shall no more rule
            over the nations.                     --Ezek. xxix.
                                                  15.
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            O thou . . . at whose sight all the stars
            Hide their diminished heads.          --Milton.
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   3. (Mus.) To make smaller by a half step; to make (an
      interval) less than minor; as, a diminished seventh.
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   4. To take away; to subtract.
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            Neither shall ye diminish aught from it. --Deut. iv.
                                                  2.
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   Diminished column, one whose upper diameter is less than
      the lower.

   Diminished scale, or Diminishing scale, a scale of
      gradation used in finding the different points for drawing
      the spiral curve of the volute. --Gwilt.

   Diminishing rule (Arch.), a board cut with a concave edge,
      for fixing the entasis and curvature of a shaft.

   Diminishing stile (Arch.), a stile which is narrower in one
      part than in another, as in many glazed doors.

   Syn: To decrease; lessen; abate; reduce; contract; curtail;
        impair; degrade. See Decrease.
        [1913 Webster]

WordNet (r) 3.0 (2006):

diminishing
    adj 1: becoming smaller or less or appearing to do so;
           "diminishing returns"; "his diminishing respect for her"

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "diminishing":
   abating, allaying, alleviating, assuaging, blunting, chastening,
   contractive, cushioning, dampening, damping, deadening, declining,
   decreasing, decrescendo, decrescent, deliquescent, diminuendo,
   dulling, dwindling, dying, easing, ebbing, fading, languishing,
   lessening, mitigating, on the wane, receding, reducing, reductive,
   relaxing, retiring, retreating, shrinking, sinking, softening,
   subduing, subsiding, tempering, waning