This conversation looks to have devolved to the point where everybody is just creating their own personal definition of Empire and saying "See, the Federation fits!" or "See, it doesn't fit!"
All this business about how it must have colonies or it must be evil or it must be a despotism or it must have no voluntary association is certainly people "creating their own personal definition." An empire is simply at its basic level -- I'll go Mirriam-Webster's on you here --
"an extensive group of states or countries under a single supreme authority."
Like I already said way back, the Federation is very specifically under a single supreme authority, which is Earth (where all its political and military authorities are plainly portrayed as being concentrated), although it is theoretically a grouping of "voluntary equals." Like the Delian League, like American and the countries of NATO, yes, that fits the definition of a de facto empire. [Oh, it also does have a bunch of colonies which are under that same central authority. So there's that, too. One wonders if other member worlds have their own colony networks which Starfleet is also policing.] Can't speak for the OP but AFAICS no fripperies about "cultural" imperialism are necessary.
Of course, as a fantasized sci-fi version of American empire, Trek also imports some measure of American exceptionalism which for some reason -- especially during the era that produced Trek -- had to strenuously insist that there was no empire happening and try to hide it behind a series of straw definitions or insist on cosmetics like the word "hegemony." Meh. That kind of posture was never very interesting to me, about America or about its fictional counterparts, but whatever works.
The Federation is a coalition between equal states and there's nothing in the show to suggest secession is prevented.
Nobody said the vision of the Federation in the show isn't more or less
idealized. That has nothing to do with the question of "empire." Those are different questions.
Although notice that
not permitting secession would hardly make the Federation any sort of evil despotic state. Very few real countries permit secession willy-nilly. I don't remember the question of secession ever coming up on the show, after all who would want to secede from The Good Guys?
