Rigid Body

An idealized object where the distance between any two internal points never changes, no matter what force it feels.

A rigid body is an idealized model from classical mechanics: no matter how much force is applied, the distance between any two points inside the object stays exactly the same — it never deforms at all. No object is a truly perfect rigid body in reality, but anything that deforms very little, like a metal link, a cup, or a wooden block, can be approximated this way. The payoff is a very compact description: in 3D space, a rigid body's state needs only 6 numbers — 3 for the position of a reference point (usually the center of mass) and 3 for orientation (as a rotation matrix, quaternion, or Euler angles) — that is, 6 degrees of freedom. Almost all of robotics modeling is built on this idealization: every link of an arm, a grasped object, and every “body” in MuJoCo or Isaac Sim is treated as a rigid body; objects that visibly deform, like cloth, rope, or dough, need soft-body simulation instead.

ExampleIn simulation, a gripper holding a block is completely determined by recording just its 3D position plus one quaternion — 7 numbers, corresponding to 6 degrees of freedom. A T-shirt, by contrast, needs hundreds or thousands of mesh vertices just to describe its shape.

Related

Rigid-Body Dynamics · Pose · Degrees of Freedom (DoF) · Link · Rigid-Body Simulation · Deformable-Body Simulation

Sources

Wikipedia: Rigid body
Modern Robotics (Lynch & Park), Ch.2 Degrees of Freedom of a Rigid Body