- noun an imaginary circle around an astronomical object, equidistant from the poles
- The imaginary line half-way between the poles of the Earth, the celestial poles, or the poles of other celestial objects exhibiting reasonably uniform rotation
- noun the imaginary great circle around the Earth’s surface, equidistant from the poles and perpendicular to the Earth’s axis of rotation which divides the earth into the northern hemisphere and the southern hemisphere
- The primary great circle of the planet earth, or that of another celestial body, which at all points is 90° from the geographic poles.
- noun an imaginary line running round the surface of the earth, at an equal distance from the north and South Poles