Fetching details from the dictionary...

helmet (of a warrior); headpiece
「甲」 has other readings with different meanings.
武士の甲は鉄製で非常に重かったです。
The samurai's helmet was very heavy.
歴史博物館で古い甲の展示を見ました。
I saw an old helmet at the history museum.
甲を被った武士が戦場に向かいました。
He put on his helmet and headed to battle.