Someone who rules during the absence or incapacity or minority of the country's monarch WordNet 3.0 & Open English WordNet

In a monarchy, a regent is a person appointed to govern a state pro tempore because the monarch is a minor, absent, incapacitated or unable to discharge the powers and duties of the monarchy, or the throne is vacant and the new monarch has not yet been determined. Wikipedia

Someone appointed to administer a state in the absence, incapacity, or minority of the monarch. Wikipedia Disambiguation

Governing official in the absence or incapacity of a monarch Wikidata

Representation of the monarch during a throne vacancy Wikidata

One who rules in place of the monarch, especially because the monarch is too young, absent, or disabled. Wiktionary

One who rules in place of the monarch. Wiktionary (translation)