Chai (חי)modern
The Hebrew word "alive" (ḥet-yod), numerically 18 — life; worn as an amulet.
Tradition: jewish · Period: Medieval Spain (use as emblem); common from 18th c.
Modern history: Word is ancient; its use as a standalone emblem is medieval-to-modern.
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Sources
Image: Public domain · AnonMoos · source. Please keep this credit when you use it.