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.

2. Sound modulated; melody. [R.]

--Thomson.

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.

Source: Webster's Revised Unabridged Dictionary (1913)