Source : Webster's Revised Unabridged Dictionary (1913)

Nadir \Na"dir\, n. [F., Sp., & It. nadir; all fr. Ar.
   nas[=i]ru's samt nadir, prop., the point opposite the zenith
   (as samt), in which nas[=i]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.

   2. The lowest point; the time of greatest depression.

            The seventh century is the nadir of the human mind
            in Europe.                            --Hallam.

   {Nadir of the sun} (Astron.), the axis of the conical shadow
      projected by the earth. --Crabb.

Source : WordNet®

nadir
     n 1: an extreme state of adversity; the lowest point of anything
          [syn: {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}]