Reincarnation: Being reborn, be it another world, the same world, as human, insect, plant, god, tree, or whatever. This is different from transmigration and rebirth(well, maybe not rebirth).
You choose what you get reincarnated as using THE WHEEL OF REINCARNATION! haha just kidding but this is how I think of how you choose what you reincarnate as.
Transmigration: Being reborn as another person, becoming that person. You take over the person's identity, debts, friends, family, life, and everything else, although, in some cases, no memory (i forgotđ). Often seen with system tropes. Different from reincarnation and rebirth.
Rebirth: Getting a second chance in YOUR life, going back to the past with all the knowledge, experiences and more. There is only one back in time in rebirth, else, regression. Not to be confused with reincarnation and regression.
Regression: The ability to make everything go back in time to a certain point, using an ability, or by death. Usually goes on forever, but can be finite. There is no fixed point at which everything is automatically regressed, that is a time loop. don't confuse with rebirth.(cheating bastards)
Isekai: Being sent to another world with your OWN identity, and not someone else's identity. You usually have no connection to the world you are summoned to. Often seen in Japanese novels,animes and manga.
I have probably missed some others but if someone happens to find this, tell me.