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Isekai

異世界(いせかい)(Isekai) is considered a genre of Japanese Fantasy, but I choose to consider it a ‘sub-genre’ or ‘theme’. It has gotten incredibly popular in recent years, with popularity really booming around 2017-2018^A). Currently, it stands as one of the most popular representatives of Japanese Fantasy, and has almost eclipsed Japanese Fantasy itself.
The amount of content that falls under the term ‘Isekai’ is so vast that it is difficult to figure out what exactly ‘Isekai’ is. This is why I choose to call it a ‘sub-genre’ or ‘theme’^B).

Definition

The defining feature of Isekai is the concept of someone coming from our world and going to another world, usually a fantasy world of some sorts. This is typically done through reincarnation after the someone dies. For whatever reason, the most common way this someone dies in Isekai stories is by getting hit by a truck^C). This is so common in-fact, that it’s become an in-joke among fans of the genre and by casual Anime fans.
As mentioned before, the number of stories that classify as ‘Isekai’ in one form or another is so vast that it’s extremely difficult to describe what they all have in common, outside of the theme of going from one world into another.
To give an example of the wide variety, the Villainess sub-genre likely originated in Isekai. Where reincarnation is a common theme. But Villainess otherwise has very little in common with other Isekai stories, which are typically closer to Japanese Fantasy stories. Another aspect that differentiates Villainess from most Isekai is the fact characters will reincarnate into a work of fiction, rather than into a distinct world, which is more common in Isekai as a whole.
The variability of Isekai stories likely has to do with the simplicity of the concept, and its usefulness to the writer, which makes Isekai quite versatile. The concept is simple enough that you could write virtually anything and it would fit under Isekai, but what makes it especially useful is that the character getting Isekai’d is unfamiliar to the world they are entering. This allows the writer to explain the worldbuilding to the audience in a fairly natural way, as it makes sense that the character would want to learn about the world they’ve entered.

Trivia and Notes

I choose to call it a sub-genre rather than a genre due to the incredible variety. Isekai can vary so much it’s a bit ridiculous to consider it a single genre.

Genre, Otaku

^A) This is my own guesstimate off of personal experience. I only really got into Japanese media around 2017. While I remember there being a fair amount of Fantasy content, and a decent amount of Isekai, it was only in 2017/2018 that I perceived a massive boom in the amount of Isekai content. Afterwards it became inescapable.

^B) But for the sake of the wiki, I have it classified under ‘genre’ anyway. That and I imagine most people would be confused if it weren’t listed as a genre.

^C) I once read someone speculating online that it’s because getting hit by a truck is both common and deadly. It’s common enough that it’s not absurd to suggest someone would die in this fashion (unlike, say, falling into a volcano), and deadly enough that it’s likely to kill you instantly. The only thing in Japan that I think would come close is getting run over by a train, but that would imply suicide, which I can imagine is an implication most authors want to avoid.