"Another world"
*Fedora Materializes"
It's a genre that is not exclusive to anime altho the term has gained traction recently, altho mainly used in a negative way in the west. It's a story where a character is transported from an stablished safe and mundane setting into a fantasy one, a western example is obviously Alice in Wonderland, and in fact many Isekai works take inspiration from it.
The reason it has gained a negative connotation is that recently it has become the defacto genre for escapism for younger viewers, teenage males mainly but not exclusively, and synonimous with a lot of particular cliches and archetypes many dislike about anime, namely the main character becoming an absolute badass due to their pre trip to the other world background, having a harem, becoming the most important person ever and also a tendency for videogame terms and structure becoming entrenched with the main plot.
Which is where the thesis that Fallout 4 is an Isekai comes from personally, if you think about it for a bit you will find all of these aspects in it.
*Fedora dematerializes, hair grows white and I fall into a coma from using the forbidden technique*
I gotta go to the gym now.