LogFAQs > #961820632

https://www.serebii.net/pokemon/mainseries.shtml

The Main Series is a term often utilised by people to differentiate the core traditional Pokmon games with the various Spin-Off and App titles that were given. While many believe it a fan term, it has a root in official terms with the "Pocket Monsters Series" () being the official Japanese term for these games
These games are traditionally, albeit not exclusively, developed by GAME FREAK and help drive the Pokmon franchise forwards. They can also sometimes connect with various Spin-Off titles to get bonuses or even receive Pokmon into the Main Series titles
In recent years, there have been twists in the Main Series games that branch it out to different subseries. These are still officially counted as main series titles, but offer slight twists on the gameplay.