Dictionary.net

If a point in a plane is taken and through it are drawn two lines or axes which are then produced indefinitely both ways, the plane is evidently divided into four portions. Suppose a point taken anywhere in the plane, its position relatively to the two straight lines or axes can be defined thus: Through the point draw a line parallel to the one axis to cut the other; the line thus drawn is called the ordinate, and the intercept the abscissa. If the lengths of the abscissa and ordinate are known, the position of the point is known, provided it be known in which of the four portions of the plane it is situated. If, however, the signs + or - prefixed to the abscissa indicate that it is measured to the right or left of the fixed point, and the same signs prefixed to the ordinate indicate that it is to be measured up or down, it is plain that, the signs and magnitudes of the ordinate and abscissa being known, the position of the point is determined without ambiguity relatively to the axes. The ordinate and abscissa are called the C. of the point, the axes or lines of reference are called C. axes, and the point through which they both pass is called the Origin of C.; when the axes are at right angles to each other the C. are rectangular, when otherwise the C. are oblique. The position of a point in space may be defined by an extension of the same method with reference to three C. planes. The position of a point on the surface of a sphere may be similarly defined by arcs of two great circles which are called its Spherical C, e.g. the latitude and longitude of a place on the earth's surface (as commonly defined) are the spherical coordinates which fix its position. C. geometry is an application of algebra to geometry, based on the determination of the position of a point by means of its co-ordinates. It is sometimes called Cartesian geometry, from the name of its inventor, Des Cartes. (For Polar co-ordinates, vide Radius-Vector.