That part of harmonic distortion which represents only the third harmonic (three times the fundamental frequency) of a sine wavepure tone input to an electronic device. The third harmonic of any tone is musically an octave and a fifth above the original tone, and is easily noticeable in the output. For this reason, the MOL of analog tape recorders, for example, is often specified as that level at which third harmonic distortion reaches 3%..