APA Dictionary of Psychology

a system for locating a point in an n-dimensional space on a graph by indicating the distance of the point from a common origin along each of n axes. The most common application of the Cartesian system is one based on two dimensions, represented by two perpendicular lines that intersect at a specific spot called the origin. In such a two-dimensional Cartesian coordinate system, the horizontal line is often referred to as the x-axis and the vertical line as the y-axis. For example, the coordinate (3, 4) represents an x value of 3 and a y value of 4 on the graph; thus, from the origin one would count three units over on the x-axis and four units up on the y-axis to locate the coordinate point. [René Descartes]