Regions are areas in the Pokémon universe that are smaller parts of a nation. Each region has their own Pokémon Professor, who provides a unique set of Starter Pokémon for young Trainers. Each region also has a unique set of eight Gym Leaders, along with the regional Elite Four. In some cases, regions can share Elite Four divisions, such as Johto and Kanto.
Currently, in the main series of games, there are six known regions (Kanto, Johto, Hoenn, Sinnoh, Unova, and Kalos). Additionally, in the Pokémon side games, five other regions are currently known; Orre, Fiore, Almia, Ransei, and Oblivia. In the Pokémon anime series and manga, a group of islands called the Orange Islands have been called a region. The Orange Islands are located south of Kanto, though this region has never been seen in any game.
Like most regions, there is a theme regarding the names of towns and cities in Kanto. Most locations in Kanto are named after a color. The regional Pokémon professor is Professor Oak, who resides in Pallet Town.
The Sevii Islands are a group of islands located south of the Kanto region. They are not to be confused with the Orange Islands, another island group near Kanto or the Blue Islands near Hoenn. The Sevii Islands are accessible in Pokémon FireRed and LeafGreen after the player defeats Blaine. They serve the purpose of a major sidequest involving Bill and is also where the player can catch Pokémon from Pokémon Gold and Silver. There are nine islands in all, and each of them is accessible once the player does something special. The islanders are ferried between the islands by the Seagallop.
Johto is the second region introduced in the main game series, being featured in the Gold, Silver and Crystal games along with the Generation IV remakes. Geographically, Johto is located to the west of the Kanto region.
The theme for locations in the Johto region is either plants or something related to a plant. The regional Pokémon professor is Professor Elm, who resides in New Bark Town.
Unlike other regions, there is no theme underlying the names of locations in Sinnoh. The regional Pokémon professor is Professor Rowan, who resides in Sandgem Town, being the first professor to live in a town that is not the player's starting location.
The Battle Zone is an island located to the northeast of Sinnoh. Battle Zone is accessible in Diamond, Pearl, and Platinum after the player defeats the Pokémon League. There are a total of three major settlements and 6 routes. The islanders do not consider themselves a part of Sinnoh.
The theme for the names of Unova locations is weather, specifically clouds, since a few of the places seem to be named after types of clouds. (Nimbasa = Nimbus, Icirrus = Cirrus, for example) The regional Pokémon professor is Professor Juniper, who is the first regional professor to be female. She resides in Nuvema Town.
Orre, which is based off the Arizona desert, is located on the northwest portion of the Kanto-Johto landmass and is generally a Wild Pokémon free region in Pokémon Colosseum and Pokémon XD. In Pokémon XD a few wild Pokémon can be captured in special locations. In both games, Pokémon called Shadow Pokémon can be "captured" from other Trainers to be purified.