Is it ensure, insure, or assure?
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.
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. (See CROWN.)