nadir

A point on the celestial sphere immediately below an observer, therefore unobservable. Its opposite, the point vertically above the observer, is the zenith.

The nadir (from Arabic nazir, "opposite") is the astronomical term for the point in the sky directly below the observer, or more precisely, the point in the sky with an inclination of -90°.

By association, nadir is also used to mean the lowest point, or time of greatest adversity. In medicine, nadir can also be used to refer to the lowest blood count for a given patient in a given period of time (i.e., a patient's nadir ANC or absolute neutrophil count).

Nadir also refers to a downward-facing viewing angle of an orbiting satellite, such as is employed during remote sensing of the atmosphere.