headgear vs helmet

headgear and helmet both are nouns.

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

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

headgear (noun)	helmet (noun)
clothing for the head	a protective headgear made of hard material to resist blows
stable gear consisting of any part of a harness that fits about the horse's head	armor plate that protects the head
the hoist at the pithead of a mine