The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Axis \Ax"is\, n.; pl. Axes. [L. axis axis, axle. See Axle.]
   A straight line, real or imaginary, passing through a body,
   on which it revolves, or may be supposed to revolve; a line
   passing through a body or system around which the parts are
   symmetrically arranged.
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   2. (Math.) A straight line with respect to which the
      different parts of a magnitude are symmetrically arranged;
      as, the axis of a cylinder, i. e., the axis of a cone,
      that is, the straight line joining the vertex and the
      center of the base; the axis of a circle, any straight
      line passing through the center.
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   3. (Bot.) The stem; the central part, or longitudinal
      support, on which organs or parts are arranged; the
      central line of any body. --Gray.
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   4. (Anat.)
      (a) The second vertebra of the neck, or vertebra
          dentata.
      (b) Also used of the body only of the vertebra, which is
          prolonged anteriorly within the foramen of the first
          vertebra or atlas, so as to form the odontoid process
          or peg which serves as a pivot for the atlas and head
          to turn upon.
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   5. (Crystallog.) One of several imaginary lines, assumed in
      describing the position of the planes by which a crystal
      is bounded.
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   6. (Fine Arts) The primary or secondary central line of any
      design.
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   Anticlinal axis (Geol.), a line or ridge from which the
      strata slope downward on the two opposite sides.

   Synclinal axis, a line from which the strata slope upward
      in opposite directions, so as to form a valley.

   Axis cylinder (Anat.), the neuraxis or essential, central
      substance of a nerve fiber; -- called also axis band,
      axial fiber, and cylinder axis.

   Axis in peritrochio, the wheel and axle, one of the
      mechanical powers.

   Axis of a curve (Geom.), a straight line which bisects a
      system of parallel chords of a curve; called a principal
      axis, when cutting them at right angles, in which case it
      divides the curve into two symmetrical portions, as in the
      parabola, which has one such axis, the ellipse, which has
      two, or the circle, which has an infinite number. The two
      axes of the ellipse are the major axis and the minor
      axis, and the two axes of the hyperbola are the
      transverse axis and the conjugate axis.

   Axis of a lens, the straight line passing through its
      center and perpendicular to its surfaces.

   Axis of a microscope or Axis of a telescope, the straight
      line with which coincide the axes of the several lenses
      which compose it.

   Axes of co["o]rdinates in a plane, two straight lines
      intersecting each other, to which points are referred for
      the purpose of determining their relative position: they
      are either rectangular or oblique.

   Axes of co["o]rdinates in space, the three straight lines
      in which the co["o]rdinate planes intersect each other.

   Axis of a balance, that line about which it turns.

   Axis of oscillation, of a pendulum, a right line passing
      through the center about which it vibrates, and
      perpendicular to the plane of vibration.

   Axis of polarization, the central line around which the
      prismatic rings or curves are arranged. --Brewster.

   Axis of revolution (Descriptive Geom.), a straight line
      about which some line or plane is revolved, so that the
      several points of the line or plane shall describe circles
      with their centers in the fixed line, and their planes
      perpendicular to it, the line describing a surface of
      revolution, and the plane a solid of revolution.

   Axis of symmetry (Geom.), any line in a plane figure which
      divides the figure into two such parts that one part, when
      folded over along the axis, shall coincide with the other
      part.

   Axis of the equator, ecliptic, horizon (or other circle
      considered with reference to the sphere on which it lies),
      the diameter of the sphere which is perpendicular to the
      plane of the circle. --Hutton.

   Axis of the Ionic capital (Arch.), a line passing
      perpendicularly through the middle of the eye of the
      volute.

   Neutral axis (Mech.), the line of demarcation between the
      horizontal elastic forces of tension and compression,
      exerted by the fibers in any cross section of a girder.

   Optic axis of a crystal, the direction in which a ray of
      transmitted light suffers no double refraction. All
      crystals, not of the isometric system, are either uniaxial
      or biaxial.

   Optic axis, Visual axis (Opt.), the straight line passing
      through the center of the pupil, and perpendicular to the
      surface of the eye.

   Radical axis of two circles (Geom.), the straight line
      perpendicular to the line joining their centers and such
      that the tangents from any point of it to the two circles
      shall be equal to each other.

   Spiral axis (Arch.), the axis of a twisted column drawn
      spirally in order to trace the circumvolutions without.

   Axis of abscissas and Axis of ordinates. See Abscissa.
      [1913 Webster]

WordNet (r) 3.0 (2006):

principal axis
    n 1: a line that passes through the center of curvature of a
         lens so that light is neither reflected nor refracted; "in
         a normal eye the optic axis is the direction in which
         objects are seen most distinctly" [syn: principal axis,
         optic axis]