Definition of meridians:
(n) :
(astronomy) In full celestial meridian: a great circle passing through the poles of the celestial sphere and the zenith for a particular point on the Earth's surface.
(n) :
(astronomy) (also geography) In full terrestrial meridian: a great circle on the Earth's surface, passing through the geographic poles (the terrestrial North Pole and South Pole); also, half of such a circle extending from pole to pole, all points of which have the same longitude.
(n) :
(by extension) The place on the celestial meridian where it is crossed by the sun or a star at its highest point.
(n) :
(by extension) (figuratively) The highest or most developed point, or most splendid stage, of something; culmination, peak, zenith.
(n) :
(by extension) (figuratively, specifically) Chiefly followed by of: the middle period of someone's life, when they are at their full abilities or strength; one's prime.
(adj) :
Relating to a meridian (in various senses); meridional.