Ball-socket joint
A joint that pins two bodies at a single point but lets them rotate freely about all three axes, like a shoulder or hip.
The constraint removes the three translational degrees of freedom and keeps the three rotational ones. Bullet calls it a point-to-point constraint, other engines a spherical joint; the principle is the same: a ball sitting in its socket. Cone and twist limits can be added so the joint cannot turn beyond what a real shoulder manages.
It is the ragdoll's workhorse: shoulders and hips are nearly always modelled as ball-socket joints, while knees and elbows get hinge joints. The constraint is the simplest of all joints to set up — a single shared anchor — and is cheap to solve every time step.