An isekai story is one in which the lead characters are transferred to and become stuck or trapped in another world, one completely different from the world they lived in up until the transfer. The new world is commonly a fictional world, such as a parallel world or virtual world; other fictional world types can also apply. Once transferred, they must use the abilities and knowledge brought from their former world as well as any that may have been bestowed upon them by their brave new world. In some cases, they will seek and eventually find a way to return; in others, they will gleefully embrace their new life.

The misnomer "reverse isekai" is sometimes used to describe when characters from another world are sent to Earth, in an inaccurate usage likely born out of a mistaken notion that isekai is designed solely around transfers of earthlings. Rather, isekai is about the transfer itself; the identity of the origin and destination worlds, or the direction in which the transfer occurs, are not relevant. What defines isekai is that the characters are sent to another world, often but not always a parallel one, and that they generally cannot come back permanently without meeting a highly specific goal.

Broadly speaking, the following major characteristics, from most to least important, define the isekai theme:

1. The characters are trapped in a new world that is not the one they lived in up to then. This new world is usually unknown to them, at least in part, but that is not strictly mandatory. Generally speaking, while exceptions may apply, pure time travel does not qualify; there must be a change of location.

2. The trapping is, at least initially, permanent; there must be no quick and easy escape route back to their home world. A known way back may exist from the outset, but it requires effort, such as clearing the 100th floor of a difficult dungeon or defeating the demon lord; alternatively, it may be eventually found or developed.

3. The transfer to the isekai world was not intentional on the part of those being transferred. For example, it may have been orchestrated by a third party (malicious or otherwise), it may have been an accident, and the universe itself could be the actor responsible.

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