a term applied to some characteristic magnitude whose value, invariable as long as one and the same function, curve, surface, etc., is considered, serves to distinguish that function, curve, surface, etc., from others of the same kind or family.-specifically (conic sections), in the ellipse and hyperbola, a third proportional to any diameter and its conjugate, or in the parabola, to any abscissa and the corresponding ordinate.-the ratio of the three crystallographic axes which determines the position of any plane; also, the fundamental axial ratio for a given species.