regent
n. countablen. a person who rules a country for a short time because the real king or queen is too young, too sick, or not there.
n. a person appointed to administer a state because the monarch is a minor, absent, or incapacitated. Often used in historical contexts to describe a temporary head of state.
The prince was only five, so his mother acted as regent.
During the king's long illness, the council appointed his eldest son as regent to manage the kingdom's daily affairs.
The history of the Regency era is defined by the period when George IV served as regent while his father was deemed unfit to rule.
From Middle English regent, from Anglo-Norman regent, Middle French regent, and their source, Latin regēns (“ruling; ruler, governor, prince”), present participle of regō (“I govern, I steer”).
Often used in the construction 'act as regent' or 'appointed as regent'.