The background noise that is continuously present on electrical circuits or radio circuits due to the thermal agitation of electrons.White noise has a flat power spectral density, which is to say that it has equal power at any frequency in any given frequency band. The term white noise comes from the fact that it is analogous to white light, which is a combination of all frequencies or wavelengths in the visible light spectrum. At acceptable levels of signal-to-noise ratio (SNR), white noise takes the form of a background hiss or mild level of static noise.White noise is even desirable and often added to digital circuits, which can be so quiet during periods of voice inactivity as to fool a listener into thinking that the connection has been dropped. People find the addition of this comfort noise to be reassuring at mild levels.At unacceptable levels of SNR, white noise can overwhelm an audio signal or cause bit errors in a data transmission. White noise is often referred to as Gaussian noise, although the two are not necessarily the same, as Gaussian noise refers to the distribution of the signal values. See also Gaussian noise, noise, and SNR.