The Collaborative International Dictionary of English v.0.48:

Nadir \Na"dir\ (n[=a]"d[~e]r), n. [F., Sp., & It. nadir; all fr.
   Ar. naz[imac]ru's samt nadir, prop., the point opposite the
   zenith (as samt), in which naz[imac]r means alike,
   corresponding to. Cf. Azimuth, Zenith.]
   1. 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.
      [1913 Webster]

   2. The lowest point; the time of greatest depression.
      [1913 Webster]

            The seventh century is the nadir of the human mind
            in Europe.                            --Hallam.
      [1913 Webster]

   Nadir of the sun (Astron.), the axis of the conical shadow
      projected by the earth. --Crabb.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nadir
    n 1: an extreme state of adversity; the lowest point of anything
         [syn: nadir, low-water mark]
    2: the point below the observer that is directly opposite the
       zenith on the imaginary sphere against which celestial bodies
       appear to be projected [ant: zenith]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "nadir":
   base, baseboard, basement, bedrock, bottom, chassis, dado, foot,
   footing, foundation, frame, keel, lower strata, lowest level,
   lowest point, mopboard, shoemold, sole, toe, underlying level,
   wainscot