Harmonic distortion

Distortion is the alteration of the original shape (or other characteristic) of something. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal representing sound or a video signal representing images, in an electronic device or communication channel. Distortion is usually unwanted, and so engineers strive to eliminate or minimize it. In some situations, however, distortion may be desirable. For example, in FM broadcasting and noise reduction systems like the Dolby system, an audio signal is deliberately distorted in ways that emphasize aspects of the signal that are subject to electrical noise, then is symmetrically "undistorted" after passing through a noisy communication channel, reducing the noise in the signal. Distortion is also used as a musical effect, particularly with electric guitars.

The addition of noise or other outside signals (hum, interference) is not considered distortion, though the effects of quantization distortion are sometimes included in noise. A quality measure that explicitly reflects both the noise and the distortion is the Signal-to-noise-and-distortion (SINAD) ratio.

In telecommunication and signal processing, a noise-free system can be characterised by a transfer function, such that the output ${\displaystyle y(t)}$ can be written as a function of the input ${\displaystyle x}$ as

...
Wikipedia