This term encompasses the set of games considered to be the "core" of the Pokémon series. These are games that share mechanics with the original games released for the Game Boy, Red, Green, Blue, and Yellow. In all core series games, the player can raise a team of up to six Pokémon, battle enemy Trainers and wild Pokémon, and capture wild Pokémon by throwing Poké Balls. Each game has a Pokédex, which records the player's progress toward catching all individual species of Pokémon. Core series games are often compatible with other core series games, allowing the player to trade and sometimes battle with other players possessing their own copy of a core series game.
Not all core series games are directly compatible; the most obvious barrier is that Pokémon cannot be traded to a game that existed prior to their creation. However, with the development and release of Pokémon Home, and with the release of the first two generations for the 3DS Virtual Console, it has become possible to trade any species of Pokémon from any main series game (and the supplementary series games) to the Home app. From Home, any game that is compatible with Home can receive Pokémon in Home provided that the Pokémon is able to be traded to that game. This effectively means that Pokémon from the games Ruby and Sapphire (released 2002) onward can be traded into Home and potentially into a modern Pokémon game, provided the player has the correct hardware, though this is becoming increasingly hard to come by.
Main series games are released in sets. For every set of games, two games are released simultaneously, which differ only in their available Pokémon (known as "version exclusives"). For Gens 1 through 4, the two released games would be followed by a third version, with quality-of-life updates and often a different plot. This did not apply to remakes, which only ever had two games released. Gens 5 and 7, instead of using a third game, instead released two follow-up games considered to be sequels or rehashes of the original two games released; these usually had significantly more content than their originals. Starting with Gen 8, instead of releasing new content as a new game, it is instead released as purchasable DLC to add on to the original content, and usually comes with updates allowing for access to new Pokémon.
Links to the pages for each of the games' in-game regions, for quicker navigation. The subpages here contain encounter tables, items, and Trainers for each location within the region.