Interregnum

Literally, 'in between reigns,' interregnum refers to the period between the end of one reign and the beginning of another.

Its specific meaning will depend on the context in which it is used. E.g., in the context of the 17th Cent, "the English interregnum" would refer to the period after the execution of Charles I in 1649, and the restoration to the monarchy of Charles II in 1660.