Astronomy . the point on the celestial sphere directly beneath a given position or observer and diametrically opposite the zenith.

Astrology . the point of a horoscope opposite the midheaven

the cusp of the fourth house.

the lowest point; point of greatest adversity or despair.

noun

all-time low , base , bottom , floor , low point , record low , rock bottom , zero levelnotes:zenith derived from arabic samt ar-ras \'the way or road above one\'s head\'; zenith technically is the point directly above the observer and nadir is the point directly below