In the broader sense, a constant, unwanted stochastic interfering signal, which is subjected to random influences.
Acoustics:
Sounds with parts of all frequencies within a given range, which do not have a phase relationship with one another and whose intensity changes statistically.
Electronics:
The effect of parts of interference layered over the useful signal. A distinction is made between white noise, which contains equal parts of all frequencies, and coloured noise, in which certain frequency ranges are more predominant.