From DominionStrategy Wiki
While the concept of a deck is fundamental to the game of Dominion, the term actually refers to several related, but separate, concepts.
(The set of cards that can be bought using Black Market is called the "Black Market deck", but that is not the subject of this article.)
 Deck as draw pile
- The concept most aligned with the use of the word Deck in the rulebook is the pile of cards from which you draw. At the start of the game, your deck contains 7 Coppers and 3 Estates or Shelters, which you shuffle together. When you need to draw or reveal cards, you take the appropriate number of cards from the top of your deck. If you need to draw or reveal cards and your deck is exhausted of cards, you shuffle your discard pile to form a new deck and complete the draw/reveal action to the best of your ability.
- You are always allowed to count how many cards are remaining in your deck, but you may not look at the cards in it unless specifically instructed to (e.g. when playing Lookout, you look at the top 3 cards of your deck).
- Most cards that interact with the deck either care about the cards on the top (e.g. Thief or Navigator), although Pearl Diver looks at the bottom card, while gaining an Inn causes you to put cards in your deck and shuffle it, and you put Stash wherever you want in your deck when you shuffle.
 Deck as draw pile + discard pile
- In some instances, it is more useful to discuss the contents of both the draw pile and discard pile together, as a kind of "potential draw pile", and "deck" is often used as shorthand for this combination. For example, the typical "draw-your-deck engine" aims to draw all cards from both piles into your hand. References to deck cycling are also usually using this definition.
- Note that Philosopher's Stone specifically refers to counting the cards in your deck and discard pile, so it considers these to be two different things.
 Deck as all cards owned
- At the end of the game, according to the rules, you put all of your cards into your deck for final scoring. Thus, at this point, your deck consists of all cards that (at the moment the game ended) were:
- In your draw pile
- In your discard pile
- In your hand
- In your play area
- On one of your mats (e.g. Tavern mat, Native Village and Island mats)
- Set aside by you (e.g. by Prince, Inheritance or by being trashed while you were playing a Possession turn)
- Your score is then based on the value of the Victory and Curse cards in your deck at this point, as well as any Victory tokens collected during the game.
- Some Victory cards care about some property of cards in your deck—for example, Gardens counts how many there are, while Fairgrounds counts how many different cards there are, and all of the above are included in those counts. Distant Lands is worth 4 if it is on your tavern mat at the end of the game, but it is also included as a card in your deck at that point and contributes to the value of other Victory cards.
 Deck as draw pile + discard pile + hand + in play
- "Deck" is also sometimes used as a shorthand for the combination of these four areas, which represent all the cards that you have access to in a turn. For example, the Golden deck typically consists of five cards which will cycle between all four areas once per turn. References to deck size and deck thinning often refer to this definition as well - technically drawing a card gets it out of your "deck", but you will usually only say that you've reduced your deck size once you've either trashed the card or set it aside.
- Any discussion about deck types is essentially using this definition as well.