Interregnum describes the period between the reigns of two leaders. If you're a rebel leader, you may try to gain power during an interregnum.
Pronounce interregnum with the accent on the third syllable: "in-ter-REG-num." It comes from Latin: inter- means "between" and regnum means "kingship, dominion, rule, realm." An interregnum can be marked by no government at all, or it can refer to a different kind of leadership between two similar regimes.