THD is a way to measure the amount of harmonic distortion of a signal. It is also used to know the degree of linearity of a device measuring the amount of distortion it generates.

To measure the THD, a pure sine wave is passed through the device to be measured, knowing the original wave and its amplitude, after it has been processed, and the amplitude of each harmonic, we can calculate it.

The amount of distortion, or THD, usually depends on the input signal level: more input signal will generate more output harmonic content because the device transfer curve will become less linear.

In the following THD graphic, the same device is shown at different input levels.