interregnum (in-tuhr-REG-nuhm) noun
The period between the end of a reign and the beginning of the next; a time when there is no government. [From Latin, from inter- (between) + regnum (reign).]
"In this unprecedented interregnum Tony Blair has no authority to do anything that attempts to tie the hands of his successor."
-- Melanie Phillips; Brown's First Great Test; Daily Mail (London, UK); June 10, 2007.