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). As an "acting monarch" the regent does the job of a monarch where the real monarch is either too young, sick or cannot do what they are meant to for some other reason.

Philippe, Duke of Orléans, Regent of France.

A regent may also be voted in if there are no heirs in line to the throne, for instance, if the current monarch dieswith no surviving relatives.