All musical instruments have tonal characteristics that consist of fundamental and harmonic frequencies. The combination of fundamental and harmonic frequencies gives a musical instrument its unique sound quality or timbre (pronounced tam-ber) and allows the human ear to distinguish between different instruments (ie. a violin compared to a cello). Harmonic frequencies are also known as overtones. For example, a violin producing a fundamental frequency of 500Hz will also produce multiples of that frequency, 1000Hz, 1500Hz, etc., which are exact multiples of the fundamental frequency. The harmonics are produced at lower levels than the fundamental frequency, but they are present and distinguish the violin from other instruments.
When an audio signal is passed through a component, unwanted harmonic frequencies that were not present in the input signal are generated and added to the original signal producing what is known as harmonic distortion. THD is expressed as a percentage of the original signal, for example .05% total harmonic distortion. This means that .05% of the signal contains unwanted harmonics or distortion not present in the original audio signal.
Harmonic distortion is present in all audio components and a lower specification is better. THD is one method of comparing audio components, but generally is not the most important, because all audio components have some amount of THD and minute differences in distortion between components is usually not audible. A listening test is the best way to evaluate sound quality.