Geometry, the motion of rotation of a line about a fixed point or centre, or of any figure about a fixed axis, or upon any line or surface. Thus, the revolution of a given line about a fixed centre, generates a circle; and that of a right-angled triangle about one side, as an axis, generates a cone; and that of a semicircle, about its diameter, generates a sphere or globe, &c.

Astronomy, is the period of a star, planet, or comet, &c., or its course from any point of its orbit, till it return to the same again.