Rigid Body

Point masses are ideal constructs, brought in for simplicity in discussion. In practice, when extended bodies interact with each other and the distances between them are very large compared to their sizes, their sizes can be ignored and they may be treated as point masses.

But when you have to consider the rotation of a body about an axis, the size of the body becomes important. When you consider the rotation of a system, you generally assume that during rotation, the distances between its constituent particles remain fixed. Such a system of particles is called a rigid body.

A rigid body is one in which the separation between the constituent particles does not change with its motion.

This definition implies that the shape of a rigid body is preserved during its motion. However, like a point mass, a rigid body is also an idealization because, if you apply large forces, the distances between particles do change, may be infinitesimally. Therefore, in nature there is nothing like a perfectly rigid body. For most purposes, a solid body is good enough approximation to a rigid body. A cricket ball, a wooden block, a steel disc, even the earth and the moon could be considered as rigid bodies.