Diadem

Definition:

(n.) Originally, an ornamental head band or fillet, worn by Eastern monarchs as a badge of royalty; hence (later), also, a crown, in general.
(n.) Regal power; sovereignty; empire; -- considered as symbolized by the crown.
(n.) An arch rising from the rim of a crown (rarely also of a coronet), and uniting with others over its center.
(v. t.) To adorn with a diadem; to crown.

Compare diadem with other words:

coronet vs. diadem