The Collaborative International Dictionary of English v.0.48:

Interregnum \In`ter*reg"num\, n.; pl. Interregnums. [L., fr.
   inter between + regnum dominion, reign. See Reign, and cf.
   Interreign.]
   [1913 Webster]
   1. The time during which a throne is vacant between the death
      or abdication of a sovereign and the accession of his
      successor.
      [1913 Webster]

   2. Any period during which, for any cause, the executive
      branch of a government is suspended or interrupted.
      [1913 Webster]

WordNet (r) 3.0 (2006):

interregnum
    n 1: the time between two reigns, governments, etc.

The Devil's Dictionary (1881-1906):

INTERREGNUM, n.  The period during which a monarchical country is
governed by a warm spot on the cushion of the throne.  The experiment
of letting the spot grow cold has commonly been attended by most
unhappy results from the zeal of many worthy persons to make it warm
again.