Kanto:
It is based on and named after the real Kantō region (関東, meaning "east of the barrier") in Japan.
Johto:
The Johto region (Japanese: ジョウト地方 Johto-chihō) is a region of the Pokémon world. It is located west of Kanto, and as revealed by the radio show Sinnoh Sound, is located south of Sinnoh.
In Japan, the Kantō region is often contrasted with the Kansai region, which like Johto is to the west. However, Johto is geographically more similar to the Kansai region and the western Chūbu region combined, as well as a small part of eastern Shikoku.
Junichi Masuda has stated in his director's column that the kanji that inspired the name mean "castle palace" (城都) or "lattice-shaped palace" (条都)[1][2]; this meaning is connected to the city of Nara, which was the basis for Violet City. Johto has a very traditional Japanese theme, which is portrayed in some of its settlements, including Ecruteak City and Violet City.
Hoenn:
Hoenn is inspired by the real-world Japanese main island of Kyushu.
Hoenn is based on the southernmost parts of Japan—Kyūshū and surrounding islands, with its main island flipped counterclockwise ninety degrees with the smaller islands circulating around its southern side.
Junichi Masuda has stated that Hoenn's name signifies "abundant relations" (豊縁 hōen), as a common theme in this region is the relationships forged between Pokémon and people.[1]
Sinnoh:
Sinnoh has been said to be an island like Hoenn, rather than a peninsula. It is based on the northernmost major island of Japan, Hokkaido. Part of Kunashir Island (Japanese: 国後島 Kunashiri Island, Russian: Кунашир Kunashir), which is in dispute between Russia and Japan, is also included.
Junichi Masuda has stated that Sinnoh's name denotes "mysteries" (神奥),[1] possibly in reference to the various myths of the region.
Unova:
Although some of Unova is geographically based on New York, the creators of the region took some liberties in regards to the surrounding areas.
Unova's Japanese name, イッシュ Isshu, is derived from the Japanese word for "one variety", 一種 isshu, or １種類 isshurui. According to the game director Masuda, this is to express that although the region is diverse in species and race, they are all the same as living creatures when seen from a distance.[3]
Kalos:
According to Junichi Masuda, the name "Kalos" comes from the Greek word κάλλος, meaning beauty.[1]
Alola:
According to Shigeru Ohmori, the name "Alola" is meant to incorporate a meaning of a land that overflows with life force and supports various forms of life.[1] The name may also come from ola, Hawaiian for life, and the traditional Hawaiian greeting aloha.
Galar:
Galar may have been derived from gallant, gala (a special occasion), or Galahad. It can also come from an anagram of San-Graal, the French name of the Holy Grail.
Hisui:
Hisui may be derived from 翡翠 hisui (jade) or 祕邃 hisui (arcane), the latter of which is synonymous to Sinnoh's name origin.
Side game regions:
Orre:
The region's name may be derived from ore, as a reference to the cities and towns that are named after minerals.
Spin-off game regions:
Fiore:
Its name is Italian for "flower".
Almia:
Possibly from alma, old Italian literary term for soul
Oblivia:
From oblivion, meaning the state of being forgotten, or possibly oblivious, considering the fact that residents are unaware of the Pincher invasion.
Source: Bulbapedia
