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, gaining popularity after the release of Sword Art Online^A) and experiencing an additional boom sometime in 2017-2018^B). 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’^C).

The defining feature of Isekai is the concept of someone coming from our world and going to another world^D), 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^E). 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.

• While a footnote mentions trains and how they might be avoided due to unfortunate implications, Youjo Senki’s protagonist does die after getting hit by a train. However, it’s made explicit that he was pushed, so it manages to avoid the implication.
  • The truck is, as morbid as it sounds, the most versatile method of dying in a story such as this. Since getting pushed implies something negative about you, and any other death would be unusual. The truck doesn’t discriminate, so it doesn’t imply anything about you.
• Konosuba has an interesting cause of death, but since it’s something of a spoiler, I won’t say.