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]

Moby Thesaurus II by Grady Ward, 1.0:

141 Moby Thesaurus words for "crux":
   Calvary cross, Chinese puzzle, Christogram, Gordian knot,
   Greek cross, Jerusalem cross, Latin cross, Maltese cross,
   Russian cross, T, X, ankh, avellan cross, baffling problem,
   bench mark, brain twister, cardinal point, catch, chi, chi-rho,
   chief thing, christcross, climax, core, cornerstone, crisis,
   crisscross, critical point, cross, cross ancre, cross botonee,
   cross bourdonee, cross fitche, cross fleury, cross formee,
   cross fourchee, cross grignolee, cross moline, cross of Cleves,
   cross of Lorraine, cross patee, cross recercelee, cross-crosslet,
   crossbones, crosslet, crossword puzzle, crucifix, cruciform,
   crux ansata, crux capitata, crux decussata, crux gammata,
   crux immissa, crux ordinaria, dagger, dilemma, enigma,
   enigmatic question, essence, essential, essential matter, ex,
   exing, floorer, fork cross, fundamental, gammadion, gist, gravamen,
   great point, heart, high point, hitch, important thing,
   inverted cross, issue, jigsaw puzzle, joker, kernel, keystone,
   knot, knotty point, landmark, long cross, main point, main thing,
   material point, matter, meat, milestone, mind-boggler, mystery,
   node, nodus, nonplus, nub, nut to crack, oxymoron, papal cross,
   paradox, pectoral cross, perplexed question, perplexity, pinch,
   pith, pivot, pons asinorum, poser, potent cross, problem, purport,
   puzzle, puzzlement, puzzler, quandary, question, question mark,
   real issue, rood, rub, salient point, saltire, sine qua non,
   sixty-four dollar question, snag, sticker, stumper, substance,
   substantive point, swastika, tau, teaser, the bottom line,
   the point, thrust, tough proposition, trefled cross, turning point,
   vexed question, voided cross, why