What does "equator" mean?

Equator: (Astron.) 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. Equator of the sun or of a planet (Astron.), the great circle whose plane passes through through the center of the body, and is perpendicular to its axis of revolution. -- Magnetic equator. See Aclinic.