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.
      [1913 Webster]

            Not diminish, but rather increase, the debt.
                                                  --Barrow.
      [1913 Webster]

   2. To lessen the authority or dignity of; to put down; to
      degrade; to abase; to weaken.
      [1913 Webster]

            This doth nothing diminish their opinion. --Robynson
                                                  (More's
                                                  Utopia).
      [1913 Webster]

            I will diminish them, that they shall no more rule
            over the nations.                     --Ezek. xxix.
                                                  15.
      [1913 Webster]

            O thou . . . at whose sight all the stars
            Hide their diminished heads.          --Milton.
      [1913 Webster]

   3. (Mus.) To make smaller by a half step; to make (an
      interval) less than minor; as, a diminished seventh.
      [1913 Webster]

   4. To take away; to subtract.
      [1913 Webster]

            Neither shall ye diminish aught from it. --Deut. iv.
                                                  2.
      [1913 Webster]

   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]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

249 Moby Thesaurus words for "diminish":
   abase, abash, abate, abbreviate, ablate, abrade, abridge, abstract,
   abuse, adjust to, allay, alleviate, allow for, alter, anesthetize,
   appease, assuage, attemper, attenuate, bank the fire, bate,
   be eaten away, belittle, benumb, blunt, boil down, box in,
   bring down, bring low, chasten, cheapen, circumscribe, clip, close,
   color, compress, condense, condition, confine, constrain,
   constrict, consume, consume away, contract, control, corrode, crop,
   crumble, crush, curtail, cushion, cut, cut back, cut down, damp,
   dampen, de-emphasize, deaden, deaden the pain, debase, decline,
   decrease, deduct, deflate, degrade, deliquesce, demean, deprecate,
   depreciate, depress, derogate, desiccate, detract, detract from,
   devalue, die away, die out, discredit, dismiss, disparage,
   dispraise, dive, dock, downgrade, downplay, drain, draw in,
   drift away, droop, drop, drop off, dry up, dull, dump, dump on,
   dwindle, ease, ease matters, ease off, eat away, ebb, erode,
   extenuate, extract, fade, fade away, fall, fall away, fall off,
   file away, flag, foment, give relief, gloss over, go, go away,
   go in, hedge, hedge about, humiliate, ignore, impair,
   keep within bounds, languish, lay, leach, leaven, lenify, lessen,
   let up, lighten, limit, lop, lower, lull, make allowance for,
   melt away, mince, minify, minimize, mitigate, moderate, modify,
   modulate, mollify, move away, move off, narrow, numb, obtund, pad,
   palliate, pare, peter out, pine, play down, plummet, plunge,
   poultice, pour balm into, pour oil on, prune, pull away, purify,
   put down, qualify, recede, reduce, reduce the temperature, refine,
   regulate by, reject, relieve, remove, restrain, restrict, retire,
   retreat, retrench, retrocede, roll back, rub away, run low, sag,
   salve, scale down, sear, season, set conditions, set down,
   set limits, shorten, shrink, shrivel, simplify, sink, slacken,
   slake, slow down, slur over, smother, snape, sober, sober down,
   soft-pedal, soften, soothe, stand off, step down, stifle, straiten,
   stupe, subduct, subdue, subside, subtract, suppress, tail off,
   take away, take down, take from, tame, taper, taper off, temper,
   thin, thin out, tone down, trip up, truncate, tune down, underplay,
   varnish, vitiate, wane, waste, waste away, weaken, wear, wear away,
   weed, whitewash, widen the distance, wilt, wind down, withdraw,
   wither, wizen, write off