—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.
Etymology: ME f. OF meridien or L meridianus (adj.) f. meridies midday f. medius middle + dies day
Useful english dictionary. 2012.