Crux is a noun that refers to the central or most important part.

Our word of the day comes directly from the Latin word for ‘the cross.’ It’s not clear how it evolved into its present meaning, but it is now used to refer to the most important part of a particular matter.

Rex and Regina came from very different cultural background, but I wouldn’t say those differences were the crux of their disagreements. The main issue had to do with their political differences.