late 13c., from Old French diademe and directly from Latin diadema "cloth band worn around the head as a sign of royalty," from Greek diadema, from diadein "to bind across," from dia- "across" (see dia-) + dein "to bind," related to desmos "band," from PIE *de- "to bind." Used of the headband worn by Persian kings and adopted by Alexander the Great and his successors.