n.1 the part of the celestial sphere directly above an observer (opp. NADIR).
2 the highest point in one's fortunes; a time of great prosperity etc.
Phrases and idioms:
zenith distance an arc intercepted between a celestial body and its zenith; the complement of a body's altitude.
Etymology: ME f. OF cenit or med.L cenit ult. f. Arab. samt (ar-ra' s) path (over the head)