A meridian is a circle passing around the Earth, through both poles. It is an example of a great circle, a circle passing fully around the Earth as the equator does, and a meridian could also be described as a great circle passing through a pole. At any given point on Earth between the poles, only one meridian passes through. The prime meridian (aka Greenwich meridian), passing through Greenwich, England, is used as a reference for maps, Earth coordinates, and standardized time.
The term meridian may also be used for the celestial sphere, and for other spherical bodies, e.g., stars, planets or moons, or their surroundings.