headpiece
A decorative type of headwear that is fitted around one's head (usually being fitted near/on the forehead) and doesn't fit the traditional definition of a crown, tiara, hairband, et cetera. A headpiece can be worn with a hat though is usually shown being worn on its own. Headpieces are usually depicted as jewelry, though it can be made out of nearly anything such as cloth, machinery, and so forth.
Jeanne d'Arc from Fate and Yomi from Puzzle & Dragons are good examples of characters that wear a headpiece.