Definition of diadem noun from the Oxford Advanced Learner's Dictionary

 

diadem

 noun
noun
BrE BrE//ˈdaɪədem//
 
; NAmE NAmE//ˈdaɪədem//
 
 
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a crown, worn especially as a sign of royal power Word Origin Middle English: from Old French diademe, via Latin from Greek diadēma ‘the regal headband of the Persian kings’, from diadein ‘bind round’.
See the Oxford Advanced American Dictionary entry: diadem