Interrex

noun

1. In ancient Rome, a regent; a magistrate who governed during an interregnum.
2. An interregent, or a regent.
3. An official in Ancient Rome, who acted as single head of state during the interregnum between two consulates.
4. An equivalent regent from the death of a Polish king till the election and enthronement of his successor.
5. A similar interim ruler, CEO etc.