Regent
governing official in the absence or incapacity of a monarch
A regent is a position of government in a country where there is a monarchy (like a king or queen). They're an "acting monarch", and does the job of a monarch where the real monarch is either too young, sick or cannot do what he is meant to for some other reason.
They may also be voted in if there are no monarchs in line to the throne, for instance, if the current monarch does not have any relatives when they die.