AHC is simply an abbreviation for "Alternate History Challenge"- You explain the desired result/outcome, rather than provide the detailed point of departure, and invite fellow discussants to suggest plausible ways to alter history beforehand to achieve that outcome.
ISOT is "island in the sea of time" - basically you grab a random landmass and send to a point back in time, with all its people and an infrastructure, and let the scenario play itself out. Since it is supernatural, it is an ASB, or otherwise magical event.
SI is a "self-insert", which is you, or the author are in the place of a historical person. It assumes you play the part historically, and aren't seen as a jibbering insane idiot, immediately carted off to the mad-house. It gained more traction than an earlier term, YWUA (You Wake Up As) person x, y, or z.
Another supernatural variant involves saying a landmass, like, to pick a favorite - Britain, gets transported not in time, but across the planet to the Pacific or the southern hemisphere in the same historical era. Sometimes this is mis-named an "ISOT" because it takes the city, country, whatever out of its immediate environment. A couple times I other people used the term, "ISOP" - 'Island in the Sea of Place' because it is space, not time that the land moves through, but that never caught on. An an even more common way to signal what is happening in a thread title is to say that land X 'teleports' somewhere.
However, that is also an ASB, supernatural, magical event, and depending on a forum's rules and culture, it may not be allowed, appropriate or may be frowned upon for violating the laws of physics or not meeting an alternate history purity or plausibility standard.