A meridian, or line of longitude, in geography is an imaginary arc connecting the Earth’s North and South Poles, or a line connecting the (two) points through which the Earth’s axis of rotation passes.
A meridian, together with its antimeridian, forms the largest circle formed by the intersection of a plane through the Earth passing through its center.
The celestial meridian is the maximum circle on the celestial sphere that passes through the poles and the local zenith; corresponding maximum semicircles are also called meridians. Celestial bodies reach their maximum height on the meridian plane. The Carrington meridian is the meridian that passed through the ascending node of the solar equator on the ecliptic at 12 noon on January 1, 1864 (Universal Time); it is the reference meridian for describing solar phenomena. For the meridian telescope and the meridian circle, see Telescope.