A system used to define the position of a point in two-dimensional and three-dimensional space. Two axes at right angles to each other are used to define the position of a point in a plane. The convention is to label the horizontal axis as the x-axis and the vertical axis as the y-axis. In this case, the origin is the intersection of the axes. The ordered pair of numbers (x, y) that defines the position of a point is the coordinate pair. Each of the numbers is a co-ordinate. The numbers are also known as Cartesian co-ordinates, after the French mathematician, René Descartes.