A **rigid body** (also known as **rigid object**) is defined as an ideal (theoretical) material body such that the distances between assigned points of the same body remain always constant during motion. That is, each particle in a rigid body maintains defined the mutual distance with the other particles of the body itself, as well as being identified by its position vector, which is constant in a reference system integral with the body (which is indicated as the reference system of the body). The stiffness hypothesis can be adopted if the detectable deformations are much less than the displacements made.

So, each particle in a rigid body maintains the reciprocal distance defined with the other particles of the body itself, as well as being identified by its position vector, which is constant in a reference system integral with the body (which is referred to as the reference system of the body).

The hypothesis of rigidity can be adopted if the detectable deformations are much lower than the displacements made. In this sense, the “generalization of the body“ in terms of a “plan attached to it“ appears of great importance, so that the plane motion of a rigid body for a reference becomes the relative motion of a free plane superimposed on a fixed plane.