Define a rigid body.

A rigid body is an object that does not change its shape or size when external forces act on it.

In physics, a rigid body means all the particles within the body remain at fixed distances from each other, even when the body moves. It can move as a whole by translation or rotation, but the relative position of its particles stays the same. 

Examples include a solid sphere, a rod, or a wheel. Ideal rigid bodies do not exist in reality because all materials deform slightly under force, but the concept simplifies mechanical analysis. 

Engineers and physicists use this assumption to study motion, torque, and equilibrium without considering small deformations.