Boids
Craig Reynolds's 1986 flocking simulation in which each individual follows three local rules and flocking emerges on its own.
The name is a contraction of "bird-oid object". Each boid sees only its nearest neighbours and obeys three rules: separation (keep your distance to avoid crowding), alignment (steer the same way as the neighbours) and cohesion (move toward the local centre of mass). Nothing directs the whole — the coordinated swarm is emergent.
Reynolds presented the model at SIGGRAPH 1987 and it became an early icon of artificial life. It made its film debut in Batman Returns (1992), whose bats and penguins were boids. Three local rules yielding believable collective motion remain the basis of swarms in games and film, and a direct ancestor of broader steering behaviors.