The branch of mathematics that combines calculus with the concept of a tensor and is therefore concerned with rates of change of quantities having more than one direction associated with them. Tensor calculus is very useful in formulating and performing calculations in the general theory of relativity. Einstein formulated the general theory in terms of tensor calculus because it is a very natural and convenient way of expressing invariance of physical quantities under coordinate transformations, which is a key feature of general relativity theory. See also Riemannian geometry. Compare vector calculus.