What does Distortion refer to?

An undersized change in a wave form or signal. A distortion is the alteration of the original shape (or other characteristic) of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice. In some fields, however, distortion may be desirable; such is the case with electric guitar distortion. The addition of noise or other extraneous signals (hum, interference) is not considered to be distortion, though the effects of quantization distortion are sometimes considered noise. A quality measure that explicitly reflects both the noise and the distortion is the Signal-to-noise-and-distortion (SINAD) ratio.