crux

1. a puzzling or difficult problem : an unsolved question
2. an essential point requiring resolution or resolving an outcome
3. a main or central feature (as of an argument)
4. ordeal of critics : a difficult problem for critics
5. the most important part of something (such as a problem, issue, puzzle, etc.) —usually + of

1. the central part or aspect of something under consideration

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Urban Dictionary is community-written and may contain offensive or explicit language.

1. The most [difficult] part. Used in many extreme [sports] to [define] the area of most dificulty.
2. p33ning [people] in the face with [burst] fire [famas]
3. [Anything] [hard] to do, or [accomplish].
4. to poop is to crux, especially an emergency poop from [drinking] too [much] [coffee] or mofe. the poop itself can also be referred to as crux.

1. matter
2. crucial
3. thereof
4. stuff
5. import
6. essence
7. shipping
8. section
9. breakage
10. weft
11. gist
12. topic
13. contents
14. theme
15. keynote
16. key
17. center
18. item
19. subject
20. thread