The point on an aerial photograph directly below the aircraft at the time of the exposure. This is the point from which radial displacement projects. On true vertical photography this is also the PP of the photograph.
The point directly below in the direction of the center of the earth. For spacecraft the arbitrarily defined 'down' direction. This is usually defined as the direction pointing toward the center of the primary body. Opposite of nadir is zenith.
Nadir is the lowest point to which blood counts drop after chemotherapy. When referring to the nadir, doctors are usually referring to the nadir of a patient's white blood cell count, which usually occurs about 7 to 10 days after receiving chemotherapy.
The lower intersection of the prime vertical and the meridian, or the point on the celestial sphere directly beneath the observer, opposite the zenith. The nadir is in the horizonal coordinate system what the Imum coeli or lower midheaven is to the ecliptic coordinate system; the two cannot be used interchangeably. Source: Astro Index