Bayer matrix
The recursively defined threshold matrix that ordered dithering compares against, built so the thresholds are spread out as widely and clump as little as possible.
The matrix is built recursively: a 2×2 doubles to 4×4, 8×8 and so on by a fixed formula that offsets each sub-quadrant by a constant. The point is that the n² thresholds are arranged so neighbouring pixels rarely draw neighbouring values — the pattern lands at high spatial frequency, where the eye minds it least. Each cell supplies a threshold the pixel value is weighed against in ordered dithering.
It is named after Bryce Bayer of Kodak, who described it in 1973 — the same person who gave his name to the colour-filter mosaic on camera sensors, though that is a wholly separate invention. The Bayer matrix dominated dithering on 8-bit hardware and in early image routines, and survives in shaders to hide colour banding at next to no cost.