Defined: The definition of crux is the central point of something, or is something that seems impossible to solve. An example of a crux is the most important beliefs of a religion. An example of a crux is a Rubik’s cube. NOUN
The basic, central, or critical point or feature: the crux of the matter; the crux of an argument. A puzzling or apparently insoluble problem.
–The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Crux is a term I’ve used at least since college. I can’t remember if I heard it or read it; likely heard it. But my usage was always along the lines of a central or critical point–“the crux of the situation.”