The equator is the intersection of the Earth's surface with the plane perpendicular to the Earth's axis of rotation and containing the Earth's center of mass. In simpler language, it is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth into a Northern Hemisphere and a Southern Hemisphere. The equators of other planets and astronomical bodies are defined analogously.
- Equator, great circle around the Earth that is everywhere equidistant from the geographic poles and lies in a plane perpendicular to the Earth’s axis. This geographic, or terrestrial, Equator divides the Earth into the Northern and Southern hemispheres and forms the imaginary reference line on the Earth’s surface from which latitude is reckoned; in other words, it is the line with 0° latitude. In astronomy, the celestial equator is the great circle in which the plane of the terrestrial Equator intersects the celestial sphere; it consequently is equidistant from the celestial poles. When the Sun lies in its plane, day and night are everywhere of equal length, a twice-per-year occurrence known as equinox.
- Equator, Encyclopædia Britannica, Inc.