crash helmet vs helmet

crash helmet and helmet both are nouns.

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

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

crash helmet: a padded helmet worn by people riding bicycles or motorcycles; protects the head in case of accidents
helmet: a protective headgear made of hard material to resist blows

crash helmet (noun)	helmet (noun)
a padded helmet worn by people riding bicycles or motorcycles; protects the head in case of accidents	a protective headgear made of hard material to resist blows
	armor plate that protects the head