THD-Total Harmonic Distortion - definition

Total Harmonic Distortion is simply known as THD. It is the amount of distortion or interference that an electrical or audio transmission has, to put it simply.
The ideal output of an electrical instrument or audio system is a signal that is unadulterated and pure. However, in practical applications, a number of variables may contribute undesired harmonic frequencies to the signal. These harmonics have the potential to corrupt the signal's original waveform.
By comparing the total power of all harmonics to the power of the original signal, THD may be used to calculate the degree of distortion in a signal. Typically, a percentage is used to indicate it. Higher THD values represent greater distortion and perhaps worse audio or electrical performance, whereas lower THD values suggest less distortion and better signal quality.
So, THD-Total Harmonic Distortion is basically a measurement of how clean and undistorted a sound or electrical signal is. Lower numbers are preferable, whereas greater values indicate more distortion.
THD is indicated with a percentage "%" sign.

i.e.: A Logitech Z623 sound system has a <0.5% THD