Rigid body
An idealised body that never deforms, whose entire state fits in a position, an orientation and its two velocities.
Assuming an object is unchangeable — the distance between any two of its points never varies — is the simplification that makes real-time game physics possible. Instead of millions of particles you need six degrees of freedom: three for position, three for rotation. Motion follows the Newton–Euler equations, where force produces acceleration and torque produces angular acceleration through the inertia tensor.
Almost every physics engine — Box2D, Bullet, PhysX, ODE — is at heart a rigid-body solver: it integrates new velocities, detects collisions and resolves the contacts with impulses so bodies do not sink into each other. For cloth, jelly or ragdolls you either join several rigid bodies with constraints or switch to a soft-body model. A chain of rigid bodies linked by joints is exactly what a ragdoll is.