helmet vs casque

helmet and casque both are nouns.

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

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

helmet: armor plate that protects the head
casque: (15-16th century) any armor for the head; usually ornate without a visor

Other hyponyms of helmet include armet, basinet, heaume, cabasset, morion, pickelhaube, salade, sallet, ocular helmet.

helmet (noun)	casque (noun)
a protective headgear made of hard material to resist blows	(15-16th century) any armor for the head; usually ornate without a visor
armor plate that protects the head