In shogi, Flatfish (olive flounder, 平目 hirame) is a Central Rook (Ranging Rook) opening.
It characteristically uses the Flatfish castle (平目囲い or ヒラメ囲い hiramegakoi) instead of the more usual Mino castle.
See also
Central Rook Central Rook Silver Horns Cheerful Central Rook Ranging Rook
References