Harmonic
Frequencies that lie between one harmonic and another harmonic are called interharmonics. For example, 1f is an interharmonic between the fundamental frequency (or first harmonic) and the second harmonic. Interharmonics is a term invented, or at least standardised, by the international electronics industry about 1999, particularly for use in electrical energy. However, interharmonic frequencies also occur naturally in other radiant energies, for example music or any type of sound.