in geometry, the straight line in a plain figure, about which it revolves, to produce or generate a solid. Thus, if a semicircle be moved round its diameter at rest, it will generate a sphere, the axis of which is that diameter.

astronomy, is an imaginary right line supposed to pass through the centre of the earth and the heavenly bodies, about which they perform their diurnal revolutions.

conic sections, a right line dividing the section into two equal parts, and cutting all its ordinates at right angles.

mechanics. The axis of a balance is that line about which it moves, or rather turns about. Axis of oscillation, is a right line parallel to the horizon, passing through the centre about which a pendulum vibrates.

Axis in Peritrochium, one of the six mechanical powers, consisting of a peritrochium or wheel concentric with the base of a cylinder, and moveable together with it about its axis.

optics, is that particular ray of light coming from any object which falls perpendicularly on the eye.

architecture. Spiral axis, is the axis of a twisted column drawn spirally in order to trace the circumvolutions without. Axis of the Ionic capital, is a line passing perpendicularly through the middle of the eye of the volute.

Axis of a Vessel, is an imaginary right line passing through the middle of it perpendicularly to its base, and equally distant from its sides.

botany, is a taper column placed in the centre of some flowers or catkins, about which the other parts are disposed.

anatomy, the name of the second vertebra of the neck; it hath a tooth which goes into the first vertebra, and this tooth is by some called the axis.