Today it means: The overthrow of a government that is considered an illegitimate regime by means of an external force (especially military might), and its replacement with a new government according to the concept of political legitimacy promoted by that force. Full definition →
Widely believed to have been coined by U.S. President Bill Clinton and popularized by his Presidential successor George W. Bush in reference to the regime of Saddam Hussein. Use of the term dates back to 1925.
Etymology from Wiktionary, CC BY-SA 4.0.