Distortionmeter (or more precisely distortion factor meter) is an electronic measuring instrument which displays the amount of distortion added to the original signal by an electronic circuit.
Harmonic distortion 
Distortion is equivalent to adding harmonics to a signal. When a purely sinusoidal signal is distorted, a series of harmonics is superimposed on the original signal.
If the input is
The normalized output is
The Total harmonics distortion (THD) is defined as the ratio of the harmonics to the fundamental;  i.e.,
This ratio can be given in dB or in percentage.
The instrument 
The distortionmeter is actually a levelmeter with two switchable parallel circuits at the input. The first circuit measures the total signal at the output of a system. (For low distortion levels this is almost equal to fundamental). That is calibrated to 100% or equivalently to 0 dB. The second circuit is a highpass filter which attenuates the fundamental, with negligible attenuation at harmonic frequencies. So when the instrument is switched for harmonics measurement the displayed level is the level of the harmonics only.
- Reference data for Radio Engineers, Howard W.Sams & co.ITT, ISBN 0-672-21218-8, p.18-12