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