Isekai is a sub-genre that has existed for a long time. It means other world and it describes a genre of stories where the main character is transported to another world. The genre had some hits in the early 2000s with titles like Magic Knight Rayearth and Inuyasha, but only around 2010s did the genre explode in popularity, giving the fans a surge of modern isekai like Sword Art Online, Re: Zero, and Mushoku Tensei, which became a fan favorite instantly.