Cellular automaton
A grid where each cell updates by a simple rule that looks only at its neighbours — the simplicity breeds surprisingly complex behaviour.
The idea comes from Stanisław Ulam and John von Neumann in the 1940s; John Conway's Game of Life (1970) made it famous. A rule along the lines of "survive with two or three neighbours, born with exactly three" is enough to get self-running patterns out of a random starting state.
In games the technique is common for cave and dungeon generation — a few iterations smooth random noise into organic cavities — and for falling-sand simulations like Noita, where fire, water and powder are cells each with their own rule.