In mathematics the imension of a space is roughly defined as the mimimum number of coordinates needed to specify every point within it. Dimensions can be thought of as the axes in a Cartesian coordinate system, which in a three-dimensional system run left-right, up-down and forward-backward. A set of three co-ordinates on these axes, or any other three-dimensional coordinate system, specifies the position of a particular point in space. In the physical world, according to the theory of relativity the fourth dimension is time, which runs before-after. An event’s position in space and time is therefore specified if four co-ordinates are given.