I had been watching and reading a lot of Isekai styled stories before I had even known there was such a term to describe them. Chances are that you have been enjoying or suffering this genre too without even knowing it.
Definition of Isekai
Isekai is a subcategory of the Fantasy genre. It refers to any story where the main character gets transferred to another world or alternative universe. The Isekai tag can be applied to games, anime, manga and light novels. Basically, any form of storytelling that uses this form as a plot basis.
While Isekai is a basic plotline that became really popular in 2016, it is often tagged with other genres like romance and action. It is not exclusively linked to any particular medium of storytelling or country.
Examples of Isekai:
The Devil Is A Part-Timer!
Shingan no Yuusha
Koushaku Reijou no Tashinami
Are you reading any Isekai manga or watching any Isekai anime?