RhymeZone: interrex definitions

Definitions of interrex:

noun: (historical) An official in Ancient Rome who acted as single head of state during the interregnum between two consulates.
noun: (historical) An equivalent regent from the death of a Polish king till the election and enthronement of his successor.
noun: (figuratively) A similar interim ruler, CEO, etc.

(Definitions from Wiktionary)