Voronoi diagram
A partition of the plane into cells, one per seed point, where each cell holds everything closest to that seed.
Named after Georgy Voronoy (1908), though Descartes had sketched the idea earlier. Given a set of seed points, every location is assigned to its nearest seed, carving space into convex cells. The diagram is the dual of the Delaunay triangulation: connect seeds whose cells share an edge and one falls out of the other.
Games lean on it constantly — biome and region maps, crack and shatter patterns for destructible geometry, procedural texture (Worley noise is just Voronoi distance) and influence maps. Fortune's sweep-line algorithm computes it in O(n log n).