crux

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The basic, central, or critical point or feature: the crux of the matter; the crux of an argument.
2. n. A puzzling or apparently insoluble problem.

Century Dictionary and Cyclopedia

1. n. A cross. See phrases below. Specifically
2. n. The Southern Cross, the most celebrated constellation of the southern heavens. It was erected into a constellation by Royer in 1679, but was often spoken of as a cross before; there even seems to be an obscure allusion to it in Dante. It is situated south of the western part of Centaurus, east of the keel of Argus. It is a small constellation of four chief stars, arranged in the form of a cross. Its brightest star, the southernmost, is of about the first magnitude; the eastern, half a magnitude fainter; the northern, of about the second magnitude; and the western, of the third magnitude and faint. The constellation owes its striking effect to its compression, for it subtends only about 6° from north to south and still less from east to west. It looks more like a kite than a cross. All four stars are white except the northernmost, which is of a clear orange-color. It contains a fifth star of the fourth magnitude, which is very red.
3. n. The cross as an instrument of torture; hence, anything that puzzles or vexes in a high degree; a conundrum.

Wiktionary

1. n. The basic, central, or essential point or feature.
2. n. The critical or transitional moment or issue, a turning point.
3. n. A puzzle or difficulty.
4. n. The hardest point of a climb.
5. n. heraldry A cross on a coat of arms.

GNU Webster's 1913

1. n. Anything that is very puzzling or difficult to explain.

WordNet 3.0

1. n. the most important point
2. n. a small conspicuous constellation in the southern hemisphere in the Milky Way near Centaurus