1. (Science: geography) The imaginary great circle on the earth's surface, everywhere equally distant from the two poles, and dividing the earth's surface into two hemispheres.
2. (Science: astronomy) The great circle of the celestial sphere, coincident with the plane of the earth's equator; so called because when the sun is in it, the days and nights are of equal length; hence called also the equinoctial, and on maps, globes, etc, the equinoctial line.
3. (Science: astronomy) equator of the sun or of a planet, the great circle whose plane passes through through the center of the body, and is perpendicular to its axis of revolution.
Origin: L. Aequator one who equalizes: cf. F. Equateur equator. See Equate.