The Collaborative International Dictionary of English v.0.48:

Modulation \Mod`u*la"tion\, n. [L. modulatio: cf. F.
   modulation.]
   1. The act of modulating, or the state of being modulated;
      as, the modulation of the voice.
      [1913 Webster]

   2. Sound modulated; melody. [R.] --Thomson.
      [1913 Webster]

   3. (Mus.) A change of key, whether transient, or until the
      music becomes established in the new key; a shifting of
      the tonality of a piece, so that the harmonies all center
      upon a new keynote or tonic; the art of transition out of
      the original key into one nearly related, and so on, it
      may be, by successive changes, into a key quite remote.
      There are also sudden and unprepared modulations.
      [1913 Webster]

   4. (Electronics) The alteration of hte amplitude, intensity,
      frequency, or phase (of the carrier wave of a radio
      signal) at intervals, so as to represent information to be
      transmitted.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

modulation
    n 1: a musical passage moving from one key to another [syn:
         transition, modulation]
    2: (electronics) the transmission of a signal by using it to
       vary a carrier wave; changing the carrier's amplitude or
       frequency or phase
    3: rise and fall of the voice pitch [syn: intonation,
       modulation, pitch contour]
    4: a manner of speaking in which the loudness or pitch or tone
       of the voice is modified [syn: modulation, inflection]
    5: the act of modifying or adjusting according to due measure
       and proportion (as with regard to artistic effect)

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "modulation":
   AM, FM, PM, abatement, about-face, accommodation, adaptation,
   adjustment, allayment, alleviation, alteration, amelioration,
   amplitude modulation, apostasy, arrangement, assuagement,
   betterment, blunting, break, calming, change, change of heart,
   changeableness, constructive change, continuity, conversion,
   damping, deadening, defection, degeneration, degenerative change,
   demulsion, deterioration, deviation, difference, diminution,
   discontinuity, divergence, diversification, diversion, diversity,
   double sideband, dulcification, dulling, easing, falling-off,
   fitting, flip-flop, gradual change, harmonization, hushing,
   improvement, inflection, instrumentation, intonation,
   intonation pattern, leniency, lessening, letdown, letup,
   lightening, loosening, lulling, melioration, mitigation,
   modification, mollification, orchestration, overthrow,
   pacification, palliation, phrasing, pitch, pitch accent,
   preparation, qualification, quietening, quieting, radical change,
   re-creation, realignment, redesign, reduction, reform, reformation,
   relaxation, remaking, remission, renewal, reshaping, resolution,
   restructuring, reversal, revival, revivification, revolution,
   setting, shift, side frequency, sideband, slackening, softening,
   solution, soothing, speech tune, subduement, sudden change,
   suprasegmental, suspension, switch, tempering, tone, tone painting,
   total change, tranquilization, transcription, transition, turn,
   turnabout, upheaval, variation, variety, violent change,
   worsening