A type of non-Euclidean geometry proposed by the German mathematician George Friedrich Bernhard Riemann (1826–66) in 1854. It is ideally suited to describing the curvature of space–time in general relativity theory. The Riemann tensor or Riemann curvature tensor is a quantity that characterizes the curvature of curved space or curved space–time (i.e. it measures the extent to which the geometry of the space or space–time deviates from Euclidean geometry). See also tensor calculus.