First use: George Washington's First Inaugural Address
April 30, 1789
The preservation of the sacred fire of liberty, and the destiny of the republican model of government, are justly considered as deeply, perhaps as finally staked, on the experiment entrusted to the hands of the American people.
That's the best definition of the American Experiment that I have ever seen.
McFly is right: it's not intended to be permanent in the institutional sense, but mutable. Its flexibility will keep it whole. Its ability to grow will prevent it from calcifying.