n. & adj.
n. 1 a circle passing through the celestial poles and zenith of any place on the earth's surface. 2 a a circle of constant longitude, passing through a given place and the terrestrial poles. b the corresponding line on a map. 3 archaic the point at which a sun or star attains its highest altitude. 4 prime; full splendour.
adj. 1 of noon. 2 of the period of greatest splendour, vigour, etc.
[ ME f. OF meridien or L meridianus (adj.) f. meridies midday f. medius middle + dies day ]