The invisible deck magic trick is a classic effect where a card is selected using a mental deck which means that any random card is chosen by the spectator from an imaginary deck. The spectator is then asked to shuffle the selected card back into the imaginary deck and lose it in there. This trick can also be done with a physical deck to make sure that the card is somehow not unconsciously suggested or mentally planted in the mind of the spectator by the magician.
Once the card is completely lost in the imaginary deck, the magician takes out a physical deck from his pocket where all the cards are face down with only one card face up and that is the spectators card. This trick is a classic which is used by almost all magicians in the world including David Blaine and Chris Angel.