One note - there was no hard term limit for US Presidents until the passage of the 22nd Amendment to the Constitution in 1951. Prior to that there was only the custom of a two-term limit, set by Washington's refusal to run for a third term in 1796. The Amendment was passed at least partially in reaction to FDR successfully winning four terms to the office.

@Jurp: Balderdash, of course there was. It's called an election every four years and there are more than sufficient examples, with or without the 22nd amendment, of Presidents who failed re-election attempts: Ford, Carter, Taft, Cleveland, Pierce, Van Buren, and J.Q. Adams, plus others who lost support of their party and declined to run and embarrass themselves, such as both Johnsons.

@Pieter Geerkens: Elections are not a term limit, because an incumbent has to be really unpopular not to be re-elected. Consider how FDR managed to win four terms, or reflect on the closeness of the recent US election.

@jamesqf The president's term is over on a certain date (March 4 until the passage of the 20th Amendment, Jan. 20 since then). If he gets re-elected, that's a new term. (In other words, there's a difference between "the limit of the term" - which is a hard limit, four years - and "the limit on how many terms," which indeed until the 22nd Amendment was in principle unlimited.)

Likewise, compare the 68+ years of Elizabeth, 63+ years for Victoria, and George III's near 60 years to Roosevelt's mere 12 years and a bit. All three of those long reigns overlap the American Republic's existence.

@PieterGeerkens: In American English, in common usage, the singular/plural distinction which you describe simply does not exist.

@Pieter Geerkens: And every one (excluding a couple who died during their 2nd term) of those US Presidents, other than FDR, CHOSE not to run for more than two terms because of the custom established by George Washington. even though there was no legal barrier to doing so prior to the 20th Amendment.

@PieterGeerkens In American English, the phrase "term limits" is the plural form of "term limit", not a limit on a different thing. The term limit of the President, and the term limit of the Governor of California, considered together are a pair of term limits. The U.S. President has a term limit of 2 terms. 4 years is the President's term length, or term duration.

While your answer is overall very good, the Soviet Union, Maoist China and the Khmer Rouge were certainly not mob rule. They were very much authoritarian.

This is a geat summary. I think this latter part is a bit incorrect in terms of absolute monarchies though (it does not detract from your overall well made points though). They may not have been absolute by the 1700s, but they had been previously. In England and France it was the commoners/middle class that forced the gradual restrictions on the absolute power of the monarchy. E.g. in England from the Magna Carter onwards and in France from events like the (much later) Tennis Court Oath. In England especially it was often excessive Royal taxation for things like wars that was resented.

"Athens practiced a political system of legislation and executive bills. Participation was far from open to all residents, but was instead limited to adult, male citizens (i.e., not a foreign resident, regardless of how many generations of the family had lived in the city, nor a slave, nor a woman), who "were probably no more than 30 percent of the total adult population"." en.wikipedia.org/wiki/Athenian_democracy

@RonJohn I'm guessing he probably just means that the revolutions that brought them into power were at least partially brought about or sparked by mob rule, then quickly consolidated into authoritarianism. Personally I don't know, one way or the other, whether that was the original spark, but I'm guessing that's probably what he means by it.

@Acccumulation: A mob of citizen aristocrats is still a mob. Would you deny that a lynch mob is a mob because it excluded blacks, and rather was formed exclusively of violently inclined whites? The larger the legislative body the more difficult it is for one, or even a few, principled individuals to oppose it. The plebiscite nature of Athenian democracy, of the citizenry as assembled in the agora, is what it is.