Geography .

a great circle of the earth passing through the poles and any given point on the earth's surface.

the half of such a circle included between the poles.

Astronomy . the great circle of the celestial sphere that passes through its poles and the observer's zenith.

a point or period of highest development, greatest prosperity, or the like.

(in acupuncture) any of the pathways in the body along which vital energy flows.

of or pertaining to a meridian.

of or pertaining to midday or noon

the meridian hour.

of or indicating a period of greatest prosperity, splendor, success, etc.

noun

nadir

noun

acme , apex , apogee , crest , culmination , extremity , high noon , high-water mark , peak , pinnacle , zenith , crown , height , summit , top , circle , climax , curve , midday , noon