Refers to protective body covering, especially traditional Japanese armor (samurai armor). Also used for Western-style armor in historical or fantasy contexts.
甲冑 is a more formal or literary term for armor and helmet as a set.
Etymology
The kanji 鎧 is a phono-semantic compound: 金 (metal) + 豈 (phonetic, also suggesting 'to cover'). The reading よろい is a native Japanese word of uncertain origin.
