Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

interregnum
     n : the time between two reigns, governments, etc.
     [also: {interregna} (pl)]