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.
      
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            Wisdom is the principal thing.        --Prov. iv. 7.
      
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   2. Of or pertaining to a prince; princely. [A Latinism]
      [Obs.] --Spenser.
      
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   {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.
      
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Challenge \Chal"lenge\, n. [OE. chalenge claim, accusation,
   challenge, OF. chalenge, chalonge, claim, accusation,
   contest, fr. L. calumnia false accusation, chicanery. See
   {Calumny}.]
   1. An invitation to engage in a contest or controversy of any
      kind; a defiance; specifically, a summons to fight a duel;
      also, the letter or message conveying the summons.
      
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            A challenge to controversy.           --Goldsmith.
      
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   2. The act of a sentry in halting any one who appears at his
      post, and demanding the countersign.
      
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   3. A claim or demand. [Obs.]
      
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            There must be no challenge of superiority.
                                                  --Collier.
      
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   4. (Hunting) The opening and crying of hounds at first
      finding the scent of their game.
      
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   5. (Law) An exception to a juror or to a member of a court
      martial, coupled with a demand that he should be held
      incompetent to act; the claim of a party that a certain
      person or persons shall not sit in trial upon him or his
      cause. --Blackstone
      
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   6. An exception to a person as not legally qualified to vote.
      The challenge must be made when the ballot is offered. [U.
      S.]
      
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   {Challenge to the array} (Law), an exception to the whole
      panel.

   {Challenge to the favor}, the alleging a special cause, the
      sufficiency of which is to be left to those whose duty and
      office it is to decide upon it.

   {Challenge to the polls}, an exception taken to any one or
      more of the individual jurors returned.

   {Peremptory challenge}, a privilege sometimes allowed to
      defendants, of challenging a certain number of jurors
      (fixed by statute in different States) without assigning
      any cause.

   {Principal challenge}, that which the law allows to be
      sufficient if found to be true.
      
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