(physics) A random signal (or process) with a flat power spectral density; a signal with a power spectral density that has equal power in any band, at any centre frequency, having a given bandwidth.
Noise that contains components at all frequencies, at least within the frequency band of interest. It is called "white" by analogy to white light, which contains light at all the visible frequencies. In the audible spectrum, white noise is a hiss or a roar, such as that produced when a television set is tuned to a channel over which no station is broadcasting.