From The Collaborative International Dictionary of English v.0.48 [gcide]:
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}.
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