In more general situations, when the material is being deformed in various directions at different rates, the strain (and therefore the strain rate) around a point within a material cannot be expressed by a single number, or even by a single vector. In such cases, the rate of deformation must be expressed by a tensor, a linear map between vectors, that expresses how the relative velocity of the medium changes when one moves by a small distance away from the point in a given direction. This strain rate tensor can be defined as the time derivative of the strain tensor, or as the symmetric part of the gradient (derivative with respect to position) of the velocity of the material.