In general usage, noise can be considered data without meaning; that is, data that is not being used to transmit a signal, but is simply produced as an unwanted by-product of other activities. In Information Theory, however, noise is still considered to be information.
When speaking of noise in relation to sound, most often it is defined as meaningless sound of greater than usual volume. Thus, a loud activity may be referred to as noisy. However, conversations of other people may be called noise for people not involved in any of them.
See:
See also: list of noise topics.
Unrelated to the preceding is Noise (Goidelic mythology)