headdress vs chapeau

headdress and chapeau both are nouns.

Word	Noun	Adjective	Verb	Adverb
headdress	Yes	No	No	No
chapeau	Yes	No	No	No

As nouns, chapeau is a hyponym of headdress; that is, chapeau is a word with a more specific, narrower meaning than headdress:

headdress: clothing for the head
chapeau: headdress that protects the head from bad weather; has shaped crown and usually a brim

headdress (noun)	chapeau (noun)
clothing for the head	headdress that protects the head from bad weather; has shaped crown and usually a brim