From Bulbapedia, the community-driven Pokémon encyclopedia.
An epicode (short for episode code) is a manner in which an episode is referred to on Bulbapedia. As episodes are released weekly in Japan, and weekly for part of the year in the rest of the world, page moves would happen quite frequently as articles were retitled for their English release– not to mention if an episode title were misinterpreted.
A commonly-accepted system was therefore developed to identify the individual episodes easily. Starting with the first episode, which was labeled as EP001, this numeric format continued throughout the episodes of the original series. After this, however, the episode counts diverged. The first episode of the second series of the anime, Advanced Generation, to some is considered just a continuation of the count from that of the original series, while to others a separate count is established. On Bulbapedia, this episode is considered AG001, and likewise, the first episode in the Diamond & Pearl series is considered DP001. Episodes of the series of specials aired during Weekly Pokémon Broadcasting Station receive the epicode of H, for Hōsō, which they were referred to prior to their dubbing as Pokémon Chronicles episodes.
Epicodes are not just for episodes of the anime, however. The chapters of Pokémon Special feature them as well.
Anime without epicodes
Several special episodes lack epicodes, usually due to the fact that they are released under extraordinary circumstances.