When MC takes over the body of an existing person and can use it as their own.
If MC possesses someone in a different world from their own, it's Transmigration instead. If MC possesses the body of a newborn child, it's Reincarnation. If the original owner of the body is still in there, it's Sharing A Body. If the original owner takes over MC's body at the same time, it's Body Swap. If MC takes over the body of their own younger self, it's Second Chance.