Floyd–Steinberg dithering
A 1976 error-diffusion algorithm that renders grey tones on a one-colour printer or screen by spreading the rounding error into neighbouring pixels.
Robert Floyd and Louis Steinberg published the method in 1976. The idea is simple and clever: when a grey value is rounded to black or white it leaves an error, and that error is pushed onto the still-unprocessed neighbours — 7/16 to the right, then 3/16, 5/16 and 1/16 below. Over an area the errors cancel out, and the eye reads a greyscale that is really just black and white dots.
The result became the standard for dithering on everything from dot-matrix and early laser printers to black-and-white screens, because it looks markedly better than a fixed grid. The drawback is the characteristic "worms" in flat tones. Newer methods such as blue-noise dithering avoid them, but Floyd–Steinberg is still the one people reach for first.