White noise
A random signal with equal energy per hertz across the whole spectrum — which is exactly why the ear hears it as bright and hissy.
The name is borrowed from light: every frequency in equal measure, the way white light contains every colour. Each sample is independent of the last, so the signal has no pattern and no memory. It sounds trebly because each octave spans twice as many hertz as the one below, so the top octaves get most of the energy. Pink noise falls 3 dB per octave and sounds even to the ear, which is why loudspeakers get measured with that instead.
In synthesizers the noise generator is the source of everything without a pitch: snare rattle, hi-hat sizzle, wind and surf. Filtered, it becomes breath; fed into a sample and hold, it becomes random melodies. In digital audio a faint layer of it is used as dither, added on purpose to hide a worse error.