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)