In common use, the word noise means any unwanted sound. In both analog electronics and digital electronics, Noise (electronics) is an unwanted perturbation to a wanted signal; it is called noise as a generalisation of the audible noise heard when listening to a weak radio transmission. Signal noise is heard as acoustic noise if played through a loudspeaker; it manifests as 'Noise (video)' on a television or video image. Noise can block, distort, change or interfere with the meaning of a message in both human and electronic communication.

In signal processing or computing it can be considered unwanted data without meaning; that is, data that is not being used to transmit a Signaling (telecommunication), but is simply produced as an unwanted by-product of other activities. "Signal-to-noise ratio" is sometimes used informally to refer to the ratio of useful information to false or irrelevant data in a conversation or exchange, such as off-topic posts and spamming in internet forum and other online communities. In information theory, however, noise is still considered to be information. In a broader sense, film grain or even advertisements encountered while looking for something else can be considered noise. In biology, noise can describe the variability of a measurement around the mean, for example transcriptional noise describes the variability in gene activity between cells in a population.

In many of these areas, the special case of thermal noise arises, which sets a fundamental lower limit to what can be measured or signaled and is related to basic physical processes at the molecular level described by well-established thermodynamics considerations, some of which are expressible by simple formulae.