Real Dictionary

nadir
The nadir of something such as someone's career or the history of an organization is its worst time.
singular noun: In astronomy, the nadir is the point at which the sun or moon is directly below you, on the other side of the earth. Compare zenith.

nadir in American English
that point of the celestial sphere directly opposite to the zenith and directly below the observer
noun: the point on the celestial sphere directly beneath a given position or observer and diametrically opposite the zenith

nadir in British English
noun: the point on the celestial sphere directly below an observer and diametrically opposite the zenith