Isshu was the Unova region's japanese name (it's the second region to have it's name changed in the pokemon series, it was changed due to not fitting the "O" naming theme of all the other regions, Kanto Johto Hoenn Sinnoh Orre Alola)
the other region was Sinnoh, which was originally called "SHINoh" as a refference to Shintoism (was changed to remove religious refferences)