Rigid body approximation
The rigid body approximation is an approximation whereby dry solid bodies are treated as rigid bodies, whence they are capable of translation and rotation, but they remain rigid, in the sense that they do not change in shape or size. Specifically, the distance between any two marked points in the body remains unchanged.
The rigid body approximation is useful when we want to ignore or abstract away issues of shape deformation. This is so even though many of the forces (normal force, friction) do occur due to these deformations. As long as these forces do not change the shapes of the bodies to a significant extent, the rigid body approximation is still useful for exploring the geometry of what is happening.
Also related is the point mass approximation.