Definition of: meridian

(mə·rid′ē·ən) noun
1. Obs. Noontime; midday.
2. The highest or culminating point of anything; the zenith: the meridian of life.
3. Astron. A great circle passing through the poles and zenith of the celestial sphere at any point; the celestial meridian.
4. Geog. a A great circle drawn from any point on the earth's surface and passing through both poles. b The half–circle so drawn between the poles, called a meridian of longitude, or geographic meridian.
5. A line on a surface of revolution in the same plane as its axis.
—adjective
1. Of or pertaining to noonday: meridian heat.
2. Pertaining to or at the highest or culminating point; brightest: meridian fame.
3. Of or pertaining to a meridian. [<OF meridien <L meridianus <meridies noon, south <medidies <medius middle + dies day]