Rigid body:
A rigid body is a system of particles (an assembly of a large number of particles), in which the distance of the inter-particle remains unchanged when it is acted upon by an external force or torque.
The rigid body is a system of mass points and the distances between all pairs of points remain unaltered throughout the motion.
So the shape of the rigid body remains the same during its motion. This motion may be translational or rotational or the combination of the two.