From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]
From WordNet (r) 3.0 (2006) [wn]:
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]