n. In mathematics: In the conic sections, that part of a transverse axis which lies between its vertex and a perpendicular ordinate to it from a given point of the conic. Thus (fig. 1), in the parabola PAC, AM, the part of the axis AB cut off by the ordinate PM, is the abscissa of the point P. In the system of Cartesian coördinates, a certain line used in determining the position of a point in a plane.