Merriam-Webster's Word of the Day for August 31, 2008 is:

nadir • \NAY-deer\  • noun
1 : the point of the celestial sphere that is directly opposite the zenith and vertically downward from the observer *2 : the lowest point

Example sentence:
Ironically, the high point of the novel occurs when the protagonist reaches her nadir, for only then does she arouse our empathy and emotional involvement.

Did you know?
"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," which names 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.

*Indicates the sense illustrated in the example sentence.