A measurement of how much an input signal is distorted by an electronic or acoustical device at its output.
Basically, THD is determined by sending a stimulus signal into a device and measuring the amount of harmonics in the output signal. It is expressed as a percentage; the lower the THD, the more accurately the device reproduces the signal.
A common variation is THD+N (total harmonic distortion plus noise), which takes the inherent noise of the electronic circuitry into account. This is still expressed as a percentage, and the lower the number, the better.