crux

Etymology

From Latin crux (“cross, wooden frame for execution”), from the Proto-Indo-European *(s)ker- (“to turn, to bend”). Doublet of cross.

noun

1. 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.
2. The critical or transitional moment or issue, a turning point.
   The mad certitude of the ogre, Abel Tiffauges, that he stands at the crux of history and that he will be able to raise Prussia "to a higher power" (p. 180), contrasts sharply with the anxiety and doubt attendant upon most modern literary dreams. 1993, Laurence M. Porter, “Real Dreams, Literary Dreams, and the Fantastic in Literature”, in Carol Schreier Rupprecht, editor, The Dream and the Text: Essays on Literature and Language, pages 32–47
3. A puzzle or difficulty.
   What I have advanced upon this species of verse will contribute to solve a poetical problem, thrown out by Dryden as a crux to his brethren 1775, Thomas Sheridan, Lectures on the Art of Reading
   The perpetual crux of New Testament chronologists. 1860, Marian Evans (translator), The Life of Jesus, Critically Examined (originally by David Strauss)
4. (climbing) The hardest point of a climb.
   the real crux of the climb was encountered 1907, The Alpine Journal, volume 23
   The final half-mile was the crux of the climb. 1973, Pat Armstrong, "Klondike Fever: Seventy Years Too Late", in Backpacker, Autumn 1973, page 84
   Most pitches have a distinct crux, or tough spot; some have multiple cruxes. […] ¶ Climb efficiently on the "cruiser" sections to stay fresh for the cruxes. 2004, Craig Luebben, Rock Climbing: Mastering Basic Skills, The Mountaineers Books, page 179
   Continue climbing the groove; the crux is passing some vegetation on the second pitch. 2009, R. J. Secor, The High Sierra: Peaks, Passes, and Trails, Third Edition, The Mountaineers Books, page 51
5. (heraldry) A cross on a coat of arms.