Total harmonic distortion measures all components a fundamental generates, including non-harmonic ones.

Evidence

Another example of the different non-linear distortion measures is the total harmonic distortion (THD), which is a measure that contains all the signal components that the fundamental frequency generates, including noise and possible non-harmonic components. Non-harmonic frequencies are those that are not integer multiples of the fundamental frequency. These also include subharmonic components, the frequencies of which are the fundamental frequency divided by an integer. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 515

Categorization

• DEFINE: Clarifies THD and subharmonic components, distinguishing harmonic from non-harmonic distortion.