signum

Signum, signi is second declension neuter and perfectly regular, but its range is wide: a scratched mark, a seal on a document, a military standard, a constellation, a statue, and in Christian Latin a miracle.

The military sense is worth learning as idiom, because it looks nothing like "sign" in translation. Signa are the standards a legion carries, so signa ferre is "to advance", signa convertere "to wheel round", signa inferre "to attack". In the Vulgate, by contrast, signa et prodigia are the wonders that authenticate a speaker, and translating them "signs" is right but thin. The trap is that signum is not a signature - that is subscriptio - and the diminutive sigillum, "little sign", gave English both "seal" and "sigil". Family: significo, designo, insignis, signifer. English sign, signal, design, insignia, ensign; Spanish seña and señal.