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

[Middle English, from Medieval Latin aequātor (diēī et noctis), equalizer (of day and night), from Latin aequāre, to equalize. See equate.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.