The distortion of a waveform caused by the nonlinear effect of a system or component that creates harmonic frequencies that would not otherwise be present at the output. The extent to which a particular harmonic component distorts a signal is given by the ratio of its amplitude An to the amplitude of the fundamental component Af, expressed as a percentage:
The total harmonic distortion (THD) is the square root of the sum of the squares of all the individual harmonic distortions:
where the individual harmonic distortion Dn is specified by the ratio An/Af.