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, or through that of a heavenly body, about which it performs its 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 Peritrochio, one of the six mechanical powers, consisting of a peritrochium or wheel concentric with the base of a cylinder, and movable together with it about its axis.

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

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.