Background noise is any unwanted signal in whatever the field of interest—for example, for acoustic, electromagnetic, Gaussian, atmospheric, cosmic, Brownian, thermal, shot, or statistical purposes. In acoustics, it is any unwanted acoustic signal that is not of direct interest but is contaminating the wanted signal by making it more problematic to hear, understand, or process. This entry defines background noise and describes how it is measured, how it can be reduced, and how it can be positive.