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