me·rid·i·an / məˈridēən/ • n. 1. a circle of constant longitude passing through a given place on the earth's surface and the terrestrial poles. ∎  (also celestial meridian) Astron. a circle passing through the celestial poles and the zenith of a given place on the earth's surface. 2. (in acupuncture and Chinese medicine) each of a set of pathways in the body along which vital energy is said to flow. There are twelve such pathways associated with specific organs.• adj. relating to or situated at a meridian: the meridian moon. ∎ poetic/lit. of noon. ∎ poetic/lit. of the period of greatest splendor, vigor, etc.

meridian. Line running north–south, i.e. where the great circle passing through the poles reaches the earth's surface. Some meridians have been marked on church floors (e.g. Santa Maria del Fiore, Florence, and Santa Maria degli Angeli, Rome), complete with signs of the zodiac and graduations.

Bibliography

W. Papworth (1852)

Me·rid·i·an / məˈridēən/ 1. a city in southwestern Idaho, west of Boise; pop. 34,919.2. a city in eastern Mississippi; pop. 39,968.

meridian
A. † midday XIV; point of sun's or star's highest altitude XV;

B. great circle of the earth or a celestial sphere XIV; individual locality XVI; adj. XIV. — (O)F. méridien or L. merīdiānus, f. mērīdiēs, nom. f. loc. merīdiē, by dissim. from *mediei diē at midday.
So meridional XIV. — F. — late L.

meridian a circle of constant longitude passing through a given place on the earth's surface and the terrestrial poles; in astronomy, a circle passing through the celestial poles and the zenith of a given place on the earth's surface.

Recorded from late Middle English, the word comes via Old French from Latin meridianum (neuter, used as a noun) ‘noon’, from medius ‘middle’ + dies ‘day’. The use in astronomy is due to the fact that the sun crosses a meridian at noon.