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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. Given some point on Earth other than the poles themselves, only one meridian passes through, termed its local meridian. 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 is also used for defining positions on other spherical bodies, such as stars, planets or moons, or their surroundings, e.g., the meridian associated with a particular location on Mars. It is also be used for lines around the celestial sphere that correspond to the Earth's meridians: the term celestial meridian (often shortened to the meridian) refers to the particular such meridian that joins the zenith with the celestial poles, i.e., the celestial sphere's meridian that corresponds at that point in time to the local meridian. The portion of the celestial meridian above observer's the horizon is termed the upper meridian and the rest (below the horizon) is termed the lower meridian.