Stadium is a spin-off.
>Prior to Generation VI, it was standard for the Western releases of the core series games to include the label Version in their title, although this was seldom used by the Japanese releases. In Japanese, Korean, and Chinese, the series is called the Pocket Monsters Series (Japanese: ポケットモンスターシリーズ[4], Korean: 포켓몬스터 시리즈[5], Chinese: 精靈寶可夢系列 / 精灵宝可梦系列[6]), as core series games all contain the full name Pocket Monsters in their title, whereas side series and spin-off games use the abbreviation Pokémon in the title instead. However, the Japanese releases of the Pokémon Stadium series use Pocket Monsters in English subtitles, despite the titles using Pokémon in kana.
ポケモンスタジアム = Pokémon Stadium