- Line of longitude running from pole to pole, on the Earth, another solid body or the celestial sphere
- An imaginary great circle along the surface of the planet which passes through both geographic poles. All points along a given meridian have the same longitude, with the prime meridian passing through longitude 0°. Also, a great half-circle joining both geographic poles.
- An imaginary great circle along the surface of a celestial body which passes through both its geographic poles, or that joins them.
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