The most important or difficult point of something; the core issue that needs to be solved.
From Latin 'crux' meaning 'cross,' borrowed into English in the 1600s; the metaphorical meaning developed from the idea of a cross being a point where paths intersect, making it crucial.
Lawyers and detectives use 'crux' constantly because it perfectly captures the idea that complex cases have one critical intersection point—the word's association with the cross shape made it irresistible to languages solving complex mysteries.
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