a vital, basic, decisive, or pivotal point: The crux of the trial was his whereabouts at the time of the murder.
2.
a cross.
3.
something that torments by its puzzling nature; a perplexing difficulty.
Origin: 1635–45; < L: stake, scaffold, or cross used in executions, torment; figurative senses perh. < NL crux (interpretum) (commentators') torment, a difficult passage in a text; cf. crucial