The Collaborative International Dictionary of English v.0.48:

Diminution \Dim`i*nu"tion\, n. [L. diminutio, or perh. rather
   deminutio: cf. F. diminution. See Diminish.]
   1. The act of diminishing, or of making or becoming less;
      state of being diminished; reduction in size, quantity, or
      degree; -- opposed to augmentation or increase.
      [1913 Webster]

   2. The act of lessening dignity or consideration, or the
      state of being deprived of dignity; a lowering in
      estimation; degradation; abasement.
      [1913 Webster]

            The world's opinion or diminution of me. --Eikon
                                                  Basilike.
      [1913 Webster]

            Nor thinks it diminution to be ranked
            In military honor next.               --Philips.
      [1913 Webster]

   3. (Law) Omission, inaccuracy, or defect in a record.
      [1913 Webster]

   4. (Mus.) In counterpoint, the imitation of, or reply to, a
      subject, in notes of half the length or value of those the
      subject itself.

   Syn: Decrease; decay; abatement; reduction; deduction;
        decrement.
        [1913 Webster]

WordNet (r) 3.0 (2006):

diminution
    n 1: change toward something smaller or lower [syn: decline,
         diminution]
    2: the statement of a theme in notes of lesser duration (usually
       half the length of the original) [ant: augmentation]
    3: the act of decreasing or reducing something [syn: decrease,
       diminution, reduction, step-down] [ant: increase,
       step-up]

Moby Thesaurus II by Grady Ward, 1.0:

100 Moby Thesaurus words for "diminution":
   abatement, abridgment, allayment, alleviation, analgesia,
   anesthesia, anesthetizing, appeasement, assuagement, attenuation,
   attrition, blunting, calming, concavity, contraction, curtailment,
   cut, cutting, dampening, damping, de-escalation, deadening,
   debasement, decrease, decrement, decrescence, deduction, deflation,
   degradation, demotion, demulsion, depletion, depreciation,
   depression, derogation, descent, detraction, detrusion,
   diminishment, dip, disparagement, ducking, dulcification, dulling,
   dying, dying off, ease, easement, easing, extenuation, extraction,
   fade-out, falling-off, hauling down, hollowness, hushing,
   impairment, languishment, leniency, lessening, letdown, letup,
   lightening, loosening, lowering, lulling, miniaturization,
   mitigation, modulation, mollification, numbing, pacification,
   palliation, quietening, quieting, reduction, relaxation, relief,
   remedy, remission, retraction, retrenchment, sagging, salving,
   scaling down, shortening, shrinkage, simplicity, sinking,
   slackening, softening, soothing, subduement, submergence,
   subtraction, tempering, thrusting under, tranquilization,
   truncation, weakening