sunhat vs sun helmet

sunhat and sun helmet both are nouns.

Word	Noun	Adjective	Verb	Adverb
sunhat	Yes	No	No	No
sun helmet	Yes	No	No	No

As nouns, sun helmet is a hyponym of sunhat; that is, sun helmet is a word with a more specific, narrower meaning than sunhat:

sunhat: a hat with a broad brim that protects the face from direct exposure to the sun
sun helmet: a lightweight hat worn in tropical countries for protection from the sun

Other hyponyms of sunhat include pith hat, pith helmet, topee, topi, sunbonnet.

sunhat (noun)	sun helmet (noun)
a hat with a broad brim that protects the face from direct exposure to the sun	a lightweight hat worn in tropical countries for protection from the sun