crux - a word I haven't seen for quite a while.
crux [krʌks]
(plural: cruxes, cruces ['krusiz])
1 a vital or decisive stage, point, etc. (often in the phrase the crux of the matter)

2 a baffling problem or difficulty

3 (Mountaineering) the most difficult and often decisive part of a climb or pitch

4 a rare word for: cross
[ETYMOLOGY: 18th Century: from Latin: cross]