Regent

A regent is "a person appointed to administer a state because the monarch is a minor, is absent or is incapacitated." The rule of a regent or regents is called a regency. A regent or regency council may be formed ad hoc or in accordance with a constitutional rule. "Regent" is sometimes a formal title. If the regent is holding his position due to his position in the line of succession, the compound term prince regent is often used; if the regent of a minor is his mother, she is often referred to as "queen regent".
If the formally appointed regent is unavailable or cannot serve on a temporary basis, a Regent ad interim may be appointed to fill the gap.