"I HAVE MEMORIES OF MY PAST LIFE! I HAD BEEN SENT HERE BY A FUCKING GODDESS! I WAS A FUCKING OFFICE WORKER IN JAPAN WHO DIED OF OVERWORK!"
Anyone who has either watched anime or read manga would know the term. Isekai was Japanese for another world. It was a concept in which a person gets transported into another place that is different from his current setting.
It was usually modern people who get sent into medieval or futuristic worlds with little to nothing but the clothes on their back. Some took the genre and sent the modern person into a game or book.
Before the person was just transported. But then authors began killing the protagonists and have them reincarnate instead. The difference between the two was that the ones who just were sent away could come back. While the latter were no longer able to. Even if they did, they were already a different person.