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.