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]