资料来源 : Webster's Revised Unabridged Dictionary (1913)

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.

资料来源 : WordNet®

principal axis
     n : a line that passes through the center of curvature of a lens
         so that light is neither reflected nor refracted [syn: {optic
         axis}]