the decisive or most important point at issue

the crux of the matter is that attitudes have changed

herein lies the crux of the issue

the smallest constellation (the Cross or Southern Cross), but the most familiar one to observers in the southern hemisphere. It contains the bright star Acrux, the ‘Jewel Box’ star cluster, and most of the Coalsack nebula

used with preceding Greek letter or numeral to designate a star in the constellation Crux

the star Beta Crucis