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. This distinction is made only in Japan, as dubbed episodes air in seasons (e.g.: Pokémon: Advanced is season 6, not season 1 of Advanced Generation series).
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, and those in the Best Wishes series by BW.
List of series