A Cartesian tensor, also known as a Cartesian coordinate tensor, is a mathematical object used in the field of physics and engineering to describe physical quantities in a way that is independent of the choice of coordinate system, as long as that system is Cartesian. In three-dimensional space, a Cartesian tensor can be represented with respect to a Cartesian coordinate system (x, y, z) and is described by its components.
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A Cartesian tensor has components in each orthonormal basis of a Euclidean space, related by the displayed rule under an orthogonal component change . Each index contributes one transformation matrix. The Euclidean inner product identifies upper and lower indices in orthonormal coordinates, so the distinction between covariant and contravariant components does not change this rule. An isotropic tensor has unchanged components under all proper rotations; requiring invariance under reflections as well excludes the ordinary odd-parity cases. A Cartesian second-rank tensor is the matrix case, with transformation .