nadir

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Astronomy A point on the celestial sphere directly below the observer, diametrically opposite the zenith.
2. n. The lowest point: the nadir of their fortunes.

Century Dictionary and Cyclopedia

1. n. That point of the heavens which is vertically below any station upon the earth. It is diametrically opposite to the zenith, or point of the heavens vertically above the station. The zenith and the nadir are thus the two poles of the horizon, the nadir being the inferior pole.
2. n. Hence The lowest point; the point of extreme depression.

Wiktionary

1. n. The point of the celestial sphere, directly opposite the zenith; inferior pole of the horizon; point of the celestial sphere directly under the place where we stand.
2. n. figuratively The lowest point; time of greatest depression.
3. n. astronomy The axis of a projected conical shadow; the direction of the force of gravity at a location; down.
4. n. beekeeping, archaic An empty box added beneath a full one in a beehive to give the colony more room to expand or store honey.
5. v. transitive, beekeeping To extend (a beehive) by adding an empty box at the base.

GNU Webster's 1913

1. n. That point of the heavens, or lower hemisphere, directly opposite the zenith; the inferior pole of the horizon; the point of the celestial sphere directly under the place where we stand.
2. n. The lowest point; the time of greatest depression.

WordNet 3.0

1. n. the point below the observer that is directly opposite the zenith on the imaginary sphere against which celestial bodies appear to be projected
2. n. an extreme state of adversity; the lowest point of anything