Diadem

A diadem is another word for a crown.  It is used to symbolize authority and rulership.  Other words are "tiara," "crown," and "turban."

"the tiara of a king (Ezek. 21:26; Isa. 28:5; 62:3); the turban (Job 29:14). In the New Testament a careful distinction is drawn between the diadem as a badge of royalty (Rev. 12:3; 13:1; 19:12) and the crown as a mark of distinction in private life. It is not known what the ancient Jewish “diadem” was. It was the mark of Oriental sovereigns."1

• 1.
  Easton, M. (1996, c1897). Easton's Bible dictionary. Oak Harbor, WA: Logos Research Systems, Inc.