From Bulbapedia, the community-driven Pokémon encyclopedia.
Stadium cards as of the Sun & Moon Series
A Stadium card (Japanese: スタジアム Stadium) is a card in the Pokémon TCG representing the venue in which the game is being played.
Following their introduction as a subclass of Trainer card in Gym Heroes and Gym Challenge, where they were based on the Gyms presided over by Kanto's Gym Leaders, Stadiums have expanded to alter virtually every aspect of play, from the HP of the combatants to the number of cards a player can draw.
From Diamond & Pearl through Call of Legends, Stadium cards were their own class of card, tinted in green, though they were still grouped with Trainer cards by fans and in set lists. From Black & White onwards, Stadium cards were reclassified as a subclass of Trainer cards.
Only one Stadium card may be in play at any time, and the playing of a new Stadium card causes the current one to be discarded. Players may only play one Stadium card per turn and may not play a Stadium card that has the same name as the one currently in play.
List of Stadium cards