Total harmonic distortion

Total harmonic distortion (THD): Of a signal, the ratio of (a) the sum of the powers of all harmonic frequencies above the fundamental frequency to (b) the power of the fundamental frequency.

Note 1: The THD is usually expressed in dB.

Note 2: Measurements for calculating the THD are made at the output of a device under specified conditions.

Source: from Federal Standard 1037C and from MIL-STD-188