crux

Short definition from Wiktionary

Noun

1. 1
   The basic, central, or essential point or feature.
2. 2
   The critical or transitional moment or issue, a turning point.
3. 3
   A puzzle or difficulty.
4. 4
   The hardest point of a climb.
5. 5
   A cross on a coat of arms.

Proper noun

1. 1
   A distinctive winter constellation of the southern sky, shaped like a cross. It appears in the flags of several countries in Oceania.