Definition
THD (total harmonic distortion) measures extra harmonics an amplifier adds to the original signal. It is usually quoted as a percentage at a given power, load, and bandwidth, for example 100W into 8Ω at 0.03% THD.
Lower is cleaner. Ratings of 1% THD describe a more distorted operation than ratings at 0.1% or below.
Wattage numbers without a THD figure a impossible to compare.
In a stereo system
When you compare power amplifier specs, read RMS watts together with the THD and impedance they were measured at. Two "100W" amps can differ if one is specified at 0.03% THD into 8Ω and the other at 1% THD into 4Ω.
THD is not the same as clipping. Clipping is a specific overload that flattens the waveform. THD includes milder distortion below the clip point. Audible clipping is a bigger practical risk than small differences in published THD among modern solid-state amps.
