Isekai (Japanese: 異世界, transl. "different world") is a genre of Japanese manga, anime, light novel, video game, etc. It has character(s) who go into a different world via rebirth. Whether that method is death, or randomly getting teleported is up to the writer.
There is some debate as to whether some anime such as Sword Art Online, is Isekai as while the protagonist go into a virtual world and eventually get out.