Isekai

1: A word meaning "Another World"
a. It can be any world unlike earth, often set in the medieval era and contains Hard Magic systems. Such as Sousou no Frieren or The Lord of the Rings.

2: It is often paired with Tensei, which means Reincarnation. So Isekai Tensei is Reincarnated to Another World.
2a. A great example is Tensei shitara Slime Datta Ken.

It has become an umbrella term that its meaning has become obscurity.

"Man, I wish i'm isekai'd by truck-kun."

"Frieren is an isekai."