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