Axis, in Geometry, is the straight line about which a plane figure revolves, so as to produce or generate a solid; or it is a straight line drawn from the vertex of a figure to the middle of the base. The axis of a circle or sphere is a straight line passing through the centre, and terminating at the circumference on the opposite sides. The axis of a cone is the line from the vertex to the centre of the base. The axis of a cylinder is the line from the centre of the one end to that of the other. Transverse axis, in the ellipse and hyperbola, is the diameter passing through the two foci, and the two principal vertices of the figure. In the hyperbola it is the shortest diameter, but in the ellipse it is the longest. Conjugate axis, in the ellipse and hyperbola, is the diameter passing through the centre, and perpendicular to the transverse axis. It is the shortest of all the conjugate diameters.

AXIS, in Mechanics, is a line about which a body may turn: by workmen, the term axis is generally considered to imply a cylindrical bar, around or upon which a wheel, or other body, rotates.

AXIS, in Peritrochio, a pedantic name which has been given to one of the mechanical powers, commonly called the wheel and axle.