Dynamis

Dynamis (デュナミス Dyunamisu) is the place where Z/X Extra cards are placed. It is a separate deck from the player's own deck.

Player may put up to 10 cards in his or her own Dynamis. The rule that states that you may only have up to 4 cards with the same name in a deck still applies to Dynamis. During a game, Dynamis is put into the field in face down position. However, a player is allowed to look at the contents of his or her own Dynamis during the game.

Etymology
"Dynamis" or "Dunamis" (δύναμις) is the Greek word for "power", "strength", or "ability". The base kanji for Dynamis in the title of Set 16 (神域 Shin'iki) means "sacred area" or "sanctuary", a reference to Dynamis being the place for Z/X Extra cards.