From Bulbapedia, the community-driven Pokémon encyclopedia.
A Stadium card as of the DPt era.
A Stadium card 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 onwards, Stadium cards became their own class of card, tinted in green, though they are still grouped with Trainer cards by fans and in set lists.
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