Isekai

Isekai is a Japanese word assimilated into the /tg/ lexicon from weeaboo fans of Japanese anime, manga and "light novels". Literally meaning "another world", it refers to a fantasy story in which the protagonist, either alone or alongside individuals who may become allies, antagonists, or both, is brought from "our" world to a foreign world, where they proceed to become an adventurer. Usually, plot reasons prevent them from heading home until something is taken care of - typically whatever big bad evil guy is threatening everything.

Methods of transportation are vast and varied, including but not limited to: stumbling into a portal, activating a magical mcguffin, dying and being reincarnated there, being summoned by the denizens of the world either unintentionally, accidentally or deliberately, or through some incredibly improbable turn of events (carried off by a tornado, sucked into an MMORPG, etc).

The Japanese take on isekai tends to have something of its own feel; such stories usually have much more self-aware protagonists, and tend to be what TVTropes would call "RPG Mechanicsverses" - settings where people actually treat the meta-game elements of /tg/ or /vg/ systems as in-universe facts, like Levels, Classes, Adventuring Guilds, etc.

Although most Japanese isekai stories tend to get panned on /tg/ for annoying meta-humor, serving as generic harem fantasy stuff (although ones that feature monstergirls like Deadline Summoner or 12 Beast are usually not quite as loathed), or both, it actually has a very long and respectable tradition in Western fantasy too.

L. Frank Baum's "Oz" books all made use of it. Edgar Rice Burroughs, the creator of Tarzan, wrote two isekai novel series; John Carter of Mars and Pellucidar. Hells, freaking Alice in Wonderland is technically an isekai story. There was even a Dungeons & Dragons cartoon that was an isekai!