MTG Keyword Rules — Comprehensive Rules Glossary

To discard a card, move it from its owner’s hand to that player’s graveyard.

By default, effects that cause a player to discard a card allow the affected player to choose which card to discard. Some effects, however, require a random discard or allow another player to choose which card is discarded.

If a card is discarded, but an effect causes it to be put into a hidden zone instead of into its owner’s graveyard without being revealed, all values of that card’s characteristics are considered to be undefined. If a card is discarded this way to pay a cost that specifies a characteristic about the discarded card, that cost payment is illegal; the game returns to the moment before the cost was paid (see rule 733, “Handling Illegal Actions”).