(n): a circular line on the surface of a sphere formed by intersecting it with a plane passing through the center
(n): a circle dividing a sphere or other surface into two usually equal and symmetrical parts
(n): ellipse in which the two axes are of equal length; a plane curve generated by one point moving at a constant distance from a fixed point
c1391 CHAUCER Astrol. I. §17 The middel cercle..is cleped also the weyere, equator of the day.