Equator

n. circle which is equally distant from both poles (on a heavenly body or sphere); imaginary circle dividing the Earth into northern and southern hemispheres

The equator is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole. It thus divides the Earth into a Northern Hemisphere and a Southern Hemisphere. The equators of other planets and astronomical bodies are defined analogously.
See more at Wikipedia.org...

n. (old spell.>aequator) equator, circle which is equally distant from both poles (on a heavenly body or sphere); imaginary circle dividing the Earth into northern and southern hemispheres

(n.)
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.
  

(n.)
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.
  

. Consider a plane perpendicular to the axis of spin of a rotating body passing through its centre. The equator is the great circle where the plane cuts the surface.