• 皇后こうごう — empress (wife of the emperor)
• 后妃こうひ — queen / royal consort
• 后宮こうきゅう — inner palace / imperial harem

后 originally depicts a person giving commands, based on the shape of a mouth and a seated figure. It has long been associated with leadership and royalty — particularly with women close to the throne.

皇后こうごうは 国民こくみんに 挨拶あいさつしました。
The empress greeted the people.

后宮こうきゅうには 多おおくの 女性じょせいが 住すんでいました。
Many women lived in the inner palace.