crux
/kɹʌks/ noun C1- The basic, central, or essential point or feature. · The crux of her argument was that the roadways needed repair before anything else could be accomplished.
- The critical or transitional moment or issue, a turning point.
- A puzzle or difficulty.
- The hardest point of a climb.
- A cross on a coat of arms.
Plural cruxes
From the library
But his papers were my great crux.
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