equator

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The imaginary great circle around the earth's surface, equidistant from the poles and perpendicular to the earth's axis of rotation. It divides the earth into the Northern Hemisphere and the Southern Hemisphere.
• noun A similar great circle drawn on the surface of a celestial body at right angles to the axis of rotation.
• noun The celestial equator.
• noun A circle that divides a sphere or other surface into congruent parts.

from The Century Dictionary.

• noun In astronomy, that imaginary great circle in the heavens the plane of which is perpendicular to the axis of the earth.
• noun In geography, that great circle of the earth every point of which is 90° from the earth's poles, which are also its poles, its axis being also the axis of the earth.
• noun Hence A similarly situated circle about any spherical body, or the region adjacent to it.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Geog.) 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.
• noun (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.
• noun (Astron.) the great circle whose plane passes through through the center of the body, and is perpendicular to its axis of revolution.
• noun See Aclinic.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An imaginary great circle around the Earth, equidistant from the two poles, and dividing earth's surface into the northern and southern hemisphere.
• noun A similar great circle on any sphere, especially on a celestial body, or on other reasonably symmetrical three-dimensional body.
• noun A short form of the celestial equator.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an imaginary line around the Earth forming the great circle that is equidistant from the north and south poles
• noun a circle dividing a sphere or other surface into two usually equal and symmetrical parts