Wang tiles
Square tiles with colour-coded edges that may only abut where the colours match — a 1961 mathematical object repurposed to build large, seamless, non-repeating textures and maps.
Hao Wang proposed them in 1961 to study whether a given set of tiles can tile the plane, the so-called domino problem. He conjectured that any set which tiles the plane does so periodically — which Robert Berger disproved in 1966 with an aperiodic set, and the question turned out to be undecidable in general. Fine mathematics, but the payoff for graphics came much later.
Cohen et al. showed in 2003 how a small set of edge-matching tiles can be laid out into enormous textures and terrain with no visible repetition and at low runtime cost. The idea is kin to autotiling and to the matching constraint that wave function collapse generalizes, and it drops naturally into a tilemap.