The basic, central, or critical point or feature: the crux of the matter; the crux of an argument. (noun)
A puzzling or apparently insoluble problem.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "crux" in a sentence
"Nor does the Latin word crux automatically refer to a cross while patibulum refer to the cross-beam."
"As for the main crux of your question I view commenting on blogs as inherently different from writing a blog."
"The crux is that few banks have been incentivised to voluntarily contribute comprehensive, high-quality information, limiting the scale of such efforts."