A type of noise whose frequency spectrum decreases at a rate of 6 dB per octave. (Compare to white noise and pink noise.) Obviously, this form of noise has even less high frequency content than pink noise. Low pass filtering can be used to further reduce the higher frequency content in order to produce "waterfall" sounds or bass rumbles. Even more drastic filtering (as by a lag processor) produces a noise signal whose frequency content is nearly all subsonic. This is useful as a randomly moving control voltage.
Mathematically, brown noise is a type of Brownian motion (named after its discoverer, 19th-century physicist Robert Brown). It is a "random walk", where each successive value is determined by adding a random offset to the previous value.