\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.

\Prin"ci*pal\, n.
1. A leader, chief, or head; one who takes the lead; one who
   acts independently, or who has controlling authority or
   influence; as, the principal of a faction, a school, a
   firm, etc.; -- distinguished from a subordinate, abettor,
   auxiliary, or assistant.
2. Hence: (Law)
   (a) The chief actor in a crime, or an abettor who is
       present at it, -- as distinguished from an accessory.
   (b) A chief obligor, promisor, or debtor, -- as
       distinguished from a surety.
   (c) One who employs another to act for him, -- as
       distinguished from an agent. --Wharton. --Bouvier.
       --Burrill.
3. A thing of chief or prime importance; something
   fundamental or especially conspicuous. Specifically:
   (a) (Com.) A capital sum of money, placed out at interest,
       due as a debt or used as a fund; -- so called in
       distinction from interest or profit.
   (b) (Arch. & Engin.) The construction which gives shape
       and strength to a roof, -- generally a truss of timber
       or iron, but there are roofs with stone principals.
       Also, loosely, the most important member of a piece of
       framing.
   (c) (Mus.) In English organs the chief open metallic stop,
       an octave above the open diapason. On the manual it is
       four feet long, on the pedal eight feet. In Germany
       this term corresponds to the English open diapason.
   (d) (O. Eng. Law) A heirloom; a mortuary. --Cowell.
   (e) pl. The first two long feathers of a hawk's wing.
       --Spenser. --J. H. Walsh.
   (f) One of turrets or pinnacles of waxwork and tapers with
       which the posts and center of a funeral hearse were
       formerly crowned. --Oxf. Gloss.
   (g) A principal or essential point or rule; a principle.
       [Obs.]