interregnum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The interval of time between the end of a sovereign's reign and the accession of a successor.
• noun A period of temporary suspension of the usual functions of government or control.
• noun A gap in continuity.

from The Century Dictionary.

• noun An intermission between reigns; an interval of time elapsing between the end of one reign and the beginning of the next, as in the case of a disputed or uncertain succession.
• noun Hence An intermission in any order of succession; any breach of continuity in action or influence.

from the GNU version of the Collaborative International Dictionary of English.

• noun The time during which a throne is vacant between the death or abdication of a sovereign and the accession of his successor.
• noun Any period during which, for any cause, the executive branch of a government is suspended or interrupted.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The period of time between the end of a sovereign's reign and the accession of another sovereign.
• noun A period of time during which normal executive leadership is suspended or interrupted.
• noun An intermission in any order of succession; any breach of continuity in action or influence.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the time between two reigns, governments, etc.