The point of the celestial sphere that is directly opposite the zenith and vertically downward from the observer.

Nadir is part of the galaxy of scientific words that have come to us from Arabic, a language that has made important contributions in the vocabulary of mathematics, astronomy, medicine, and chemistry. Nadir derives from an Arabic word meaning "opposite"—the opposite, that is, of the zenith, or the highest point of the celestial sphere, the one vertically above the observer. (The word zenith itself is a modification of another Arabic word that means "the way over one's head.") The English poet John Donne is first on record as having used nadir in the figurative sense of "lowest point" in a sermon he wrote in 1627.