female emperor (of Japan)
Refers specifically to a woman who has ascended the Chrysanthemum Throne in her own right, as opposed to an empress consort.
Empress Suiko is considered Japan's first female emperor.
Debate continues over the enthronement of a female emperor.
天皇 is the general term for the emperor of Japan, regardless of gender. 女性天皇 specifies a female emperor.
皇后 is the empress consort, the wife of the reigning emperor. 女性天皇 is a reigning empress in her own right.
Compound of 女性 (woman) and 天皇 (emperor). The term is modern, used in historical and political contexts to distinguish female sovereigns from male emperors.