Source : Webster's Revised Unabridged Dictionary (1913)

Principal \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.]

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.

Source : WordNet®

principal
     n 1: the original amount of a debt on which interest is
          calculated
     2: the educator who has executive authority for a school; "she
        sent unruly pupils to see the principal" [syn: {school
        principal}, {head teacher}, {head}]
     3: an actor who plays a principal role [syn: {star}, {lead}]
     4: capital as contrasted with the income derived from it [syn:
        {corpus}, {principal sum}]
     5: the major party to a financial transaction at a stock
        exchange; buys and sells for his own account [syn: {dealer}]

principal
     adj : most important element; "the chief aim of living"; "the main
           doors were of solid glass"; "the principal rivers of
           America"; "the principal example"; "policemen were
           primary targets" [syn: {chief(a)}, {main(a)}, {primary(a)},
            {principal(a)}]