The Collaborative International Dictionary of English v.0.48:
Principal \Prin"ci*pal\, a. [F., from L. principalis. See
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]
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
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.