Seilfall

Seilfall is the other world where Ryouma Takebayashi was reincarnated by the gods Gain, Kufo and Lulutia after his death on Earth. His presence in the land is to help replenish the diminished mana of the world.

Rifall
The kingdom the Gods sent Ryoma to due to the country being more tolerant of the social gap between commoners and nobility and being easier to live in compared to other countries.

Gilmar
A country that has been at war with itself for a very long time.

Trivia

 * Time seems sped up here in comparison to Earth. Shiho Jamil was shown to be wearing a sailor school uniform in a flashback, which is only a late 20th/21st century outfit in Japan. Even an "otaku" was something that wasn't really a thing until pop culture became prominent.
 * Reincarnators may accidentally screw up the legacy of others who come here. A doctor got sent a few centuries before a saint did; her wish to eradicate all sickness ended up causing all the medical knowledge to get ignored and fade away. So it's really NO wonder why this world is so underdeveloped.
 * Though the Saint proved that a reincarnation can offer up their very existence to have a wish granted. The Gain, Kufo and Lulutina were saddened, but respected the girl's wish; even if it meant her soul ceased to exist.
 * Kufo outright says this world is BORING due to the lack entertainment like on Earth. Even Ryoma had to make do with Slime research to pass the time when not training/experimenting with his abilities.
 * No libraries to share fiction and non-fiction stories and information.
 * No bands to play music for the public
 * No TV, video games, movies, comics, etc.
 * It's odd that Kufo, Lulutina and Ganfall didn't try bringing brilliant minds like Edison, Telsa, Ford and Einstein. Or even Shakespeare to improve the entertainment. All of them would have helped progress