Dither (audio)
A faint, deliberate noise added before bit depth is reduced, so the rounding error becomes steady hiss instead of distortion that tracks the music.
When a 24-bit mix goes down to the 16 bits of a CD, every sample gets rounded. With loud material the error is random enough to pass as noise, but in a long fade it becomes correlated with the signal: the tail picks up a grainy, unmistakably digital fizz that pulses along with the music. Add a little random noise before rounding and that link is broken; what remains is a constant hiss.
The usual choice is TPDF dither, with a triangular probability distribution spanning ±1 least significant bit. Vanderkooy and Lipshitz showed in the 1980s that it removes both the distortion and the audible noise modulation, at the cost of a noise floor just under 5 dB higher. The rule is to dither once, at the very end. Dithering every intermediate step just piles up hiss.
Image dithering runs on exactly the same idea: smooth noise beats visible steps.