Center of mass
The point where an object's mass can be treated as concentrated — a free body rotates around it, and the physics engine reckons torque and balance about it.
For translation, a rigid body behaves as if all its mass sat at a single point; forces that do not pass through that point also create rotation. Game physics needs the point to decide how an object tumbles when hit, whether a vehicle rolls in a corner, or how a character keeps its balance.
Engines usually compute it from the collider's shape, but developers often move it by hand — lowering a car's centre of mass makes it more stable, placing it low in a figure helps an active ragdoll stay upright. Together with the inertia tensor it fully describes how a body responds to forces.