1. The imaginary great circle around earth that is the same distance from the North and South Poles and divides the earth into the northern and southern hemispheres.
2. An imaginary great circle around an astronomical object that is everywhere the same distance from the poles.
3. In mathematics, a circle that divides a sphere or other surface into two equal parts.
4. In medicine, an imaginary line encircling a globe, equidistant from the poles.

Used in anatomical nomenclature to designate such a line on a spherical organ, dividing the surface into two approximately equal parts.