1814, "cross," from Latin crux "cross" (see cross (n.)). Figurative use for "a central difficulty," is older, from 1718; perhaps from Latin crux interpretum "a point [..]

basic or very important part of something.

Southern Cross: a small conspicuous constellation in the southern hemisphere in the Milky Way near Centaurus the most important point Crux is the smallest of the 88 modern constellations, but is one o [..]

(n) a small conspicuous constellation in the southern hemisphere in the Milky Way near Centaurus(n) the most important point

A knotty point, a difficulty. Instantia crucis means a crucial test, or the point where two similar diseases crossed and showed a special feature. It does not refer to the cross, an instrument of puni [..]

The basic, central, or essential point or feature. ''The crux of her argument was that the roadways needed repair before anything else could be accomplished.'' The critical or transitional momen [..]

(constellation) A distinctive winter constellation of the southern sky, shaped like a cross. It appears in the flags of several countries in Oceania.

sometimes used for crus

The hardest move on a pitch or the hardest pitch on a climb.