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.
W. Papworth (1852)
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.
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.