☡

☡ U+2621, ☡ CAUTION SIGN ← ☠ [U+2620] Miscellaneous Symbols ☢ → [U+2622]

Translingual

Etymology

In mathematics, first introduced by Nicolas Bourbaki in their treatise Éléments de mathématique.

Symbol

☡

1. (mathematics, computer science) Used to mark an especially difficult passage or proof in a textbook.