From the REG:
Discard
Discard is removing a card from its current location and placing it face up on the top of the discard pile. A card may be discarded from a player’s hand, draw pile, set-aside area, or Field of Play. Once a card is discarded, it cannot be reintroduced into the game unless another card's special abilities instruct this to happen. Any time a card is discarded, it returns to face value.