regency; regent
Refers to the office or person acting as ruler when the monarch is a minor, incapacitated, or absent. In Japanese history, the 摂政 (Sesshō) was a powerful regent position, often held by members of the Fujiwara clan.
During the Heian period, the Fujiwara clan monopolized the position of regent.
Because the emperor is young, the regent conducts state affairs on his behalf.