Contact manifold
The set of contact points between two touching bodies — a box on the floor touches it at four corners, not at a single point.
A single contact point is enough to detect a hit but not to hold a resting box flat — it would pivot on that one point. The contact manifold is the small set of points (often at most four) that describes the touching region, together with a shared normal. Without it, stacking and resting contact go unstable.
Engines like Box2D and Bullet keep a persistent manifold, matching points from frame to frame so the solver can "warm start" from last frame's impulses. That's the trick that lets a tower of boxes settle instead of buzzing in place.