headpiece vs helmet

headpiece and helmet both are nouns.

Word	Noun	Adjective	Verb	Adverb
headpiece	Yes	No	No	No
helmet	Yes	No	No	No

As nouns, helmet is a hypernym of headpiece; that is, helmet is a word with a broader meaning than headpiece:

headpiece (noun)	helmet (noun)
a protective helmet for the head	a protective headgear made of hard material to resist blows
the band that is the part of a bridle that fits around a horse's head	armor plate that protects the head