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space-time In relativity theory, a central concept that unifies the three space dimensions (length, breadth, and height) with time to form a four-dimensional frame of reference. Durations and rates of processes depend on the relative state of motion of the observer and the system observed. In 1907, Hermann Minkowski clarified relativity theory by describing space-time in terms of a four-dimensional geometry. Three co-ordinates of space and a time co-ordinate specify an event in space-time. A line drawn in this space represents a particle's path both in space and time. Albert Einstein incorporated this viewpoint into his theory of relativity: in the General Theory, gravity is a distortion of space-time by matter.