A sine wave is a pure tone, a single harmonic, often called the fundamental. Harmonic distortion is when higher harmonics are introduced into the original pure tone. Harmonic distortion is typically introduced by imperfect amplification and normally introduces odd harmonics ie 3rd, 5th, 7th etc. For example if you assume the pure tone is 1KHz then harmonic distortion would normally be seen at 3KHz, 5HKz, 7KHz etc. THD (Total Harmonic Distortion) is a measure of harmonic distortion and is quoted as a percent. It is the ratio of the total power contained in harmonics to the power contained in the fundamental tone. For audio amplifiers THD specs should be around 0.1%. More information can be found in most audio handbooks but more precise information can be found in any good electronic text book such as "The Art of Electronics":- www.amazon.co.uk
