Physics island
A group of bodies touching or jointed together, solved as a unit — but independently of every other group.
A physics engine doesn't solve the whole world as one system. It partitions bodies into islands: sets connected by contacts or joints. Each island is solved on its own, which makes them trivial to parallelise and lets an island where everything has come to rest sleep wholesale.
Islands are rebuilt every step with a union-find pass over the contact graph. Erin Catto's Box2D and Bullet both use them. The payoff shows in a scene of a thousand scattered crates — that's a thousand tiny islands to solve separately, not one solve chewing through a thousand bodies at once.