Source : Webster's Revised Unabridged Dictionary (1913)

Crux \Crux\ (kr[u^]ks), n.; pl. E. {Cruxes} (-[e^]z), L.
   {Cruces} (kr[udd]"s[=e]z). [L., cross, torture, trouble.]
   Anything that is very puzzling or difficult to explain. --Dr.
   Sheridan.

         The perpetual crux of New Testament chronologists.
                                                  --Strauss.

Source : WordNet®

Crux
     n 1: a small conspicuous constellation in the southern hemisphere
          in the Milky Way near Centaurus [syn: {Southern Cross},
          {Crux Australis}]
     2: the most important point [syn: {crux of the matter}]