A rigid body is an idealization of a body in which deformation is zero no matter how large an external deforming force is applied on it.
We can define a rigid body as an object in which the relative positions of its constituent particles remains unchanged when external deforming forces are applied to it.
Actually rigid body is an ideal concept as no body is perfectly rigid and every body can be deformed more or less by the application of suitable forces. But as a nearest approach to a rigid body diamond can be a perfect example because one the Mohs scale (a scale of hardness used in classifying minerals) of relative hardness, a diamond is rated as 10, which is maximum.
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