Any part of a signal that is not the true or original signal but is introduced by the communication mechanism.

A common example would be an electrical signal travelling down a wire to which noise is added by inductive and capacitive coupling with other nearby signals (this kind of noise is known as "crosstalk").

A less obvious form of noise is quantisation noise, such as the error between the true colour of a point in a scene in the real world and its representation as a pixel in a digital image.