From Bulbapedia, the community-driven Pokémon encyclopedia.
A series of the anime is a collection of episodes that corresponds roughly to a specific generation of games.
For Japanese episodes, this distinction is made instead of seasons (although there are other subdivisions, such as Episode Orange Islands and Best Wishes! Season 2). In the dub, this distinction was not properly made until Pokémon the Series: XY, which introduced a name for the series, being followed by some older series retroactively receiving similar names in home video box sets and video-on-demand releases (such as iTunes and Netflix). With the exception of Pokémon: Advanced (only when using standard season divisions), dub seasons never contain episodes from more than one series, being cut off at series divisions.
On Bulbapedia, the episodes in each series are divided from other series by their episode code. Episodes in the original series are denoted by EP, those in the Advanced Generation series by AG, those in the Diamond & Pearl series by DP, those in the Best Wishes series by BW, and those in the XY series by XY.
List of series