Isekai
"Hey guys, today I wanted to talk about the newest, hottest anime to come out this season. All right, get this: It's about a completely normal shut-in Otaku with a very specific skill set that makes him useless in the real world, who is suddenly transported to a fantasy world kinda similar to any JRPG you've ever seen where he suddenly becomes the hottest shit, and he has two jobs: Messing up any poor soul who looks at him the wrong way and getting some 2D bitches. Wait, doesn't this sound oddly familiar?"
- – Gigguk, "Isekai: The Genre that Took Over Anime"
Proof that Japan has no publishing standards or quality control. Isekai is a Japanese word assimilated into the /tg/ lexicon from the weeaboo faggots at /a/ and /jp/. Literally meaning "another world" or "parallel world", it refers to a genre in which the main characters are from "our" world and taken to a foreign world resembling some form of fantasy game, where they proceed to become adventurers. Usually, plot reasons prevent them from heading home until something is taken care of—typically whatever big bad evil guy is threatening everything—but sometimes they're stuck there forever and have to adapt as best they can. Methods of transportation are vast and varied, including but not limited to: stumbling into a portal, activating a magical McGuffin, getting run over by Truck-kun and reincarnated (Tensei in weeb, a genre isekai ate), being summoned by the denizens of the world, or the ever-popular getting your brain downloaded into your favorite MMORPG.
The term (and to a lesser extent the genre) have been kicking around the weeaboosphere for a while, but around 2015 publishers started flooding the market with insufferably awful series (with insufferably long titles) that sell both in Japan and internationally like hotcakes, no matter how bland and generic they get. This once again proves that no matter which side of the planet you're on, otaku are autistic retards with no taste. As of 2018 this seems to be tapering off: Kadokawa has banned isekai stories from their light novel competitions, fewer and fewer isekai light novels get adapted into anime each season, and parodies are becoming more and more common, making it only a matter of time before the genre hits "even the parodies are stale" levels of played out.
Isekai and /tg/
Although most isekai stories get panned on /tg/ for annoying meta-humor, generic shonen bullshit, generic fanservice bullshit, or a combination thereof (if not the characters being blatantly Mary Sues, or presenting something even more absurd), a handful of series are decent enough to merit genuine approval. Or they're tolerated because they have monstergirls. Check our anime and manga pages for the current scoop.
While isekai is a distinctly Japanese form of cancer, the basic idea of people from our world getting chucked into a fantastic world and forced to fend for themselves is practically universal and turns up moderately often in Western fantasy with the earliest example perhaps being "A Connecticut Yankee in King Arthur's Court" by Mark Twain which was published in 1889. Oddly, when this happens it tends to be rather less shit perhaps due to it being less common. L. Frank Baum's Oz series, Alice's Adventures in Wonderland by Lewis Carroll, and Edgar Rice Burroughs' Barsoom (a.k.a., John Carter of Mars) novels are iconic examples of the core premise that predate cliche fantasy, and C.S. Lewis The Chronicles of Narnia uses the plot for Christian allegory. The NeverEnding Story is the flagship modern western example, and right in the heart of the fantasy cliche storm, yet it is the purest anti-shit, either despite or because of this. Or at least, it avoids being the self-indulgent wish-fulfillment for irredeemably unlikable losers that makes Isekai so widely hated. One could make the case that The Matrix is an isekai story (it basically reverses a couple of the key tropes), though classifying it as "less shit" may not be accurate for some people. Tangential to these are stories about modern militaries (or, in one odd series of novels, part of the US East Coast) being sent back in time—although it's possible that a movie from '79 called G.I. Samurai, where a JSDF unit accidentally travels back in time and fights their own Samurai ancestors, is secretly the true forgotten granddaddy of the isekai genre, or at least dreck like GATE.
Isekai also has its influence on Old School Roleplaying; as stated above, there are plenty of pulp fantasy novels involving ordinary souls getting sucked into a strange, alien world and becoming heroic adventurers as a result. A /tg/ example that (in hindsight anyway) fits the isekai mold well is GURPS' flagship fantasy setting, which revolves around people from across the universe getting isekai'd to the planet of Yrth by an extradimensional "Banestorm" and proposes that players could stat themselves and then play as themselves on Yrth after getting deposited there by the Banestorm. Hell, Greyhawk has several deities who actually originated on other worlds - Murlynd, Saint Cuthbert and Mayaheine have all been implied to have come to Oerth from "real" Earth - whilst the Forgotten Realms was, once upon a time, hinted as being connected to Earth by various portals to different times and places; the not!Egyptian race was actually supposed to be peopled by real ancient Egyptians who had been summoned to the Realms en-masse by evil sorcerers as slave labor, only to break free of them. Then there's the D&D Cartoon, whose plot was D&D by way of Isekai. That being said, unless your DM was being really lazy, if you tried to talk in-universe about stats or levels or other meta game content like they do in Isekai stories, NPCs would and should treat you like a madman.
Reverse Isekai
Occasionally, reverse isekai plots, where supernatural elements from other dimensions have invaded the "real" world, have appeared in /tg/. D20 Modern's default for supernatural entities is that they a dropped onto Earth from another plane, "The Shadow", and can't go home (though their corpses vanish upon death). The Adventure Path Reign of Winter has a trip to World War I era Russia where the party fights Mosin-Nagants and machine gun wielding Russian soldiers, tear gas elementals and actual Grigori Rasputin.
One odd feature in Japanese Reverse-Isekais is an emphasis on how Japanese food is so much more awesome than whatever bland, flavorless food the peasants of the fantasy world have to eat. In fact, there actually is more than one anime about people from a fantasy world visiting a restaurant in modern Japan. Which in fairness: the modern world wide food distribution networks that can ship sun ripened lemons and meat to any point in the world within 24 hours is likely going to compare favorable to all but the highest fantasy fare. Even so, even the lowliest peasant would put some effort in using what they had to make food taste good; even if they couldn't afford spices, herbs were still easy enough to get a hold of, and rural cooks knew enough about how to prepare meats to make them taste good. Whereas fantasy peasants may as well be eating dry, stringy meat with a side of boiled, unseasoned vegetables and mud for dessert.