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.
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   2. The lowest point; the time of greatest depression.
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            The seventh century is the nadir of the human mind
            in Europe.                            --Hallam.
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   Nadir of the sun (Astron.), the axis of the conical shadow
      projected by the earth. --Crabb.
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