What is "principal axes of a quadric"

Principal \Prin"ci*pal\, a. [F., from L. principalis. See Prince.]

1. Highest in rank, authority, character, importance, or degree; most considerable or important; chief; main; as, the principal officers of a Government; the principal men of a state; the principal productions of a country; the principal arguments in a case.

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

2. Of or pertaining to a prince; princely. [A Latinism] [Obs.]
   --Spenser.

   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.