equator

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.
2. n. A similar great circle drawn on the surface of a celestial body at right angles to the axis of rotation.
3. n. The celestial equator.
4. n. A circle that divides a sphere or other surface into congruent parts.

Century Dictionary and Cyclopedia

1. n. In astronomy, that imaginary great circle in the heavens the plane of which is perpendicular to the axis of the earth. It is everywhere 90° distant from the celestial poles, which coincide with the extremities of the earth's axis, supposed to be produced to meet the heavens, and its axis is this produced axis. It divides the celestial sphere into the northern and southern hemispheres. During his apparent yearly course the sun is twice in the equator, in the months of March and September. Then the day and night are everywhere equal, whence the name equator.
2. n. 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. It is in the plane of the celestial equator. Our earth is divided by it into the northern and southern hemispheres. From this circle is reckoned the latitude of places both north and south.
3. n. Hence A similarly situated circle about any spherical body, or the region adjacent to it.

Wiktionary

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

GNU Webster's 1913

1. n. (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.
2. n. (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.

WordNet 3.0

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