modulation

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 ]