1580s, from Latin interregnum "an interval between two reigns," literally "between-reign," from inter "between" (see inter-) + regnum "kingship, dominion, rule, realm," related to regere "to rule, to direct, keep straight, guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule"). In the republic, it meant a vacancy in the consulate. The earlier English noun was interreign (1530s), from French interrègne (14c.).
