Distortion is usually thought of as an undesirable change in the waveform as it passes through electronic equipment or any kind of transducer, although, like noise, this is a highly subjective and context dependent judgement. 
Some sound synthesis techniques such as non-linear waveshaping use distortion to achieve their ends.  The familiar sound of the rock guitar is largely dependent on distortion techniques.  Any technique which changes the signal can be considered a form of distortion, so even a simple equalizer may be thought to distort the signal. 
Several kinds of distortion occur in electroacoustic
systems such as amplifiers, mixers, and microphones: