Source : Webster's Revised Unabridged Dictionary (1913)

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}.

            Not diminish, but rather increase, the debt.
                                                  --Barrow.

   2. To lessen the authority or dignity of; to put down; to
      degrade; to abase; to weaken.

            This doth nothing diminish their opinion. --Robynson
                                                  (More's
                                                  Utopia).

            I will diminish them, that they shall no more rule
            over the nations.                     --Ezek. xxix.
                                                  15.

            O thou . . . at whose sight all the stars Hide their
            diminished heads.                     --Milton.

   3. (Mus.) To make smaller by a half step; to make (an
      interval) less than minor; as, a diminished seventh.

   4. To take away; to subtract.

            Neither shall ye diminish aught from it. --Deut. iv.
                                                  2.

   {Diminished column}, one whose upper diameter is less than
      the lower.

   {Diminished}, 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}.

Diminish \Di*min"ish\, v. i.
   To become or appear less or smaller; to lessen; as, the
   apparent size of an object diminishes as we recede from it.

Source : WordNet®

diminish
     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 fall to under a
          hundred pounds"; "his voice fell to a whisper" [syn: {decrease},
           {lessen}, {fall}] [ant: {increase}]
     2: lessen the authority, dignity, or reputation of; "don't
        belittle your colleagues" [syn: {belittle}]