sun hat vs chapeau

sun hat and chapeau both are nouns.

Word	Noun	Adjective	Verb	Adverb
sun hat	Yes	No	No	No
chapeau	Yes	No	No	No

As nouns, chapeau is a hypernym of sun hat; that is, chapeau is a word with a broader meaning than sun hat:

sun hat: a hat with a broad brim that protects the face from direct exposure to the sun
chapeau: headdress that protects the head from bad weather; has shaped crown and usually a brim

Other hypernyms of sun hat include hat, lid.

sun hat (noun)	chapeau (noun)
a hat with a broad brim that protects the face from direct exposure to the sun	headdress that protects the head from bad weather; has shaped crown and usually a brim