Regime change can occur through conquest by a foreign power, revolution, coup d'état or reconstruction following the failure of a state. Regime change may replace all or part of the state's existing institutions, administrative apparatus, bureaucracy and other elements.
Popular use 
The transition from one political regime to another, esp through concerted political or military action - most recently seen in the regime change undergone by Tunisia.
The term has been popularized by recent US presidents. Bill Clinton and George W. Bush regularly used the term in reference to Saddam Hussein's regime in Iraq. Ronald Reagan had previously called for regime change in Libya, directing the Central Intelligence Agency (CIA) to work towards that goal.
The term regime change is sometimes erroneously used to describe a change in the government of the day.
Internal regime change 
In academic use 
In addition to the above uses, the term 'regime change' can also be used in a more general sense, particularly in academic work, to refer to a change in political institutions or laws that affect the nature of the system as a whole. For example, the end of the Bretton Woods system was a regime change in the international system, as was the repeal of the National Mandatory Speed Limit in the United States. Regime changes are often viewed as ideal opportunities for natural experiments by social scientists.
See also 
- Oxford English Dictionary, http://dictionary.oed.com, September 2007 draft
- Washington Post 20 Feb. 1987