Generally, output is a linear function of input only for a fixed portion of the transfer characteristics.
When output is not in this portion, two forms of amplitude distortion might arise Due to the additional outputs, this form of distortion is definitely unwanted in audio, radio and telecommunication amplifiers, and it occurs for more than two waves as well.
These so-called third order distortion products (third order as m+n = 3) tend to dominate the non-linear distortion of narrowband systems.
Amplitude distortion is measured with the system operating under steady-state conditions with a sinusoidal input signal.
When other frequencies are present, the term "amplitude" refers to that of the fundamental only.