Isekai (Japanese: 異世界, transl. "different world") is an anime that has a character from the real world go into a fantasy world via rebirth. Whether that method is death, or randomly getting teleported is up to the writer. Anime that are regarded as Isekai include Re:Zero , That Time I Got Reincarnated As A Slime, Overlord, No Game No Life, Log Horizon, etc.
There is some debate as to whether some anime such as Sword Art Online, is Isekai as while the protagonist is in another world it is a virtual one and eventually they get out.