nadir

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Astronomy A point on the celestial sphere directly below the observer, diametrically opposite the zenith.
• noun The lowest point.

from The Century Dictionary.

• noun That point of the heavens which is vertically below any station upon the earth.
• noun Hence The lowest point; the point of extreme depression.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun The lowest point; the time of greatest depression.
• noun (Astron.) the axis of the conical shadow projected by the earth.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.
• noun figuratively The lowest point; time of greatest depression.
• noun astronomy The axis of a projected conical shadow; the direction of the force of gravity at a location; down.
• noun beekeeping, archaic An empty box added beneath a full one in a beehive to give the colony more room to expand or store honey.
• verb transitive, beekeeping To extend (a beehive) by adding an empty box at the base.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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