sun hat vs pith helmet

sun hat and pith helmet both are nouns.

Word	Noun	Adjective	Verb	Adverb
sun hat	Yes	No	No	No
pith helmet	Yes	No	No	No

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

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

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

sun hat (noun)	pith 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