White noise is a signal or process with a flat frequency spectrum. In other words, there is equal energy in a given bandwidth at any centre frequency. In time domain, the magnitude of a white noise process has normal distribution.
In digital signal processing, almost all types of noise are often approximated with white noise since it has several useful statistical properties. For example, it is statistically uncorrelated in time with any other signal, including itself.
In practical applications few signals are exactly white noise, or even close to white noise. However, signals such as gunshot noise and thermal noise have a close to flat spectrum.
A closely related concept is pink noise which a sound that is perceptually white noise. That is, the human auditory system perceives equal magnitude on all frequencies.
See also: Statistics
White Noise is a 1985 novel by Don DeLillo.