A 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.
A distortionmeter is a level meter with two switchable parallel circuits at the input.
The second circuit is a high pass filter which removes (as much as practical) the fundamental frequency.
This can be a notch filter, one which passes all but the fundamental, with negligible attenuation at other frequencies (including whatever harmonics might be present).
The output of the filter is measured as a percentage of the fundamental, and the reported value will be the distortion value.