White noise is a random noise that contains an equal amount of energy in all frequency bands.
Hear a 5-second example of white noise (download file):
White noise is the equivalent of white light, in fact this is how it gets it's name. White light is made up of all light frequencies (colours), while white noise is made up of all audio frequencies.
White noise is used in electronic music, either directly as a sound effect or as the basis to create synthesized sounds. For example, many percussion instruments have a high component of white noise.
White noise is also used to mask other sounds. This process takes advantage of the way the human brain works — the brain is able to single out simple frequency ranges but has trouble when too many frequencies are heard at once. When white noise is present, other noises appear diminished.
White noise is available on CDs etc, marketed as a noise reducer or sleeping aid.
Main page: Noise Colours