What does the word modulation mean?

Usage examples for modulation

  1. The problem of harmony therefore in practice reduces itself to that of modulation. – Wagner's Tristan und Isolde by George Ainslie Hight
  2. Whereas, by constant attention to the manner of using the voice, since they are in constant practice, it might be perfected in its modulation. – The American Union Speaker by John D. Philbrick