Physics engine
Also said as Rigid body simulation, Solver, Havok, Box2D, PhysX
The part of an engine that moves things under forces and keeps them from passing through each other. It solves joints and contacts approximately, by nudging them in several passes each step.
It gives you behaviour nobody had to animate: things stack, topple and swing. It also gives you things nobody asked for, so any object that matters to the design needs testing against it.
Say it
“Crates and barrels go in the physics engine at 60 Hz with 8 solver passes. The player stays on a custom controller so physics can never push them through a wall.”
Where you’ve seen it
Half-Life 2 integrated the Havok physics engine into Valve's Source engine, and built its gravity gun and its physics puzzles on it.
Not to be confused with character controller
Most platformers do not put their hero in the physics engine at all. A character controller is hand-written movement that only asks physics what it is touching, because real physics feels terrible to steer.