Wisdom is the principal thing. Prov. iv. 7.

Principal axis. See Axis of a curve, under Axis. -- Principal axes of a quadric (Geom.), three lines in which the principal planes of the solid intersect two and two, as in an ellipsoid. -- Principal challenge. (Law) See under Challenge. -- Principal plane. See Plane of projection (a), under Plane. -- Principal of a quadric (Geom.), three planes each of which is at right angles to the other two, and bisects all chords of the quadric perpendicular to the plane, as in an ellipsoid. -- Principal point (Persp.), the projection of the point of sight upon the plane of projection. -- Principal ray (Persp.), the line drawn through the point of sight perpendicular to the perspective plane. -- Principal section (Crystallog.), a plane passing through the optical axis of a crystal.