The Kingdom comprises the cards in the Supply in a game of Dominion other than basic cards.
Although the basic gameplay rules are the same from one game of Dominion to the next, every game is different because a different set of Kingdom cards is chosen for each game. The set of Kingdom cards chosen in a particular game is often called the Kingdom. To date (since the release of Plunder), there are 426 distinct Kingdom card piles (not counting 37 removed cards), 10 of which are ordinarily selected for each game. The heart of the strategy of the game is therefore adapting one's play to maximally benefit from the different combinations of abilities each game's particular set of Kingdom cards makes available. Most of the Kingdom cards are Actions, but all expansions have some Treasure and/or Victory kingdom cards as well.
If one of the Kingdom cards being used in a given game is Young Witch, an eleventh Kingdom card pile is added to the supply at the beginning of the game. Knights, Castles, and split piles create situations where a single Kingdom card pile in the supply contains two or more differently-named cards. The card Black Market makes it possible to buy Kingdom cards that aren't present in the supply for that game.
Some Kingdom cards will result in other non-Kingdom cards being present in the game. If any of the selected Kingdom cards (or a card in the Black Market) has a Potion is added to the basic Treasures; if any Kingdom cards are Looters, then Ruins will be in the supply. Some Kingdom cards can let players access certain non-Supply cards, such as the Prizes from Tournament or the Traveller lines of Page and Peasant; certain Kingdom cards in Nocturne, if present in the game, cause players' starting decks to contain Heirlooms.in its cost, then
Other features can change from game to game that are not specifically dependent on the selection of Kingdom cards, such as whether Shelters, Colonies, or landscapes such as Events are used; but these are not generally considered properties of the Kingdom. The set of Kingdom cards plus additional variable elements such as these is collectively often called the Board.
Confusion over terminology
Several cards from Adventures originally referred to Kingdom cards or piles, but were changed during playtesting as many testers did not understand what Kingdom cards were. This included the cards and events that moved tokens such as Teacher and Ferry.