The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         The perpetual crux of New Testament chronologists.
                                                  --Strauss.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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