What does it mean to discard a card?

To discard a card means to move it from a player's hand to their graveyard. This action typically occurs when a player has to reduce the number of cards in their hand, often due to card effects or game rules that limit hand size. When a card is discarded, it is no longer in the player's hand and is instead placed in the graveyard, where it can be interacted with by various game mechanics.

The other choices describe different actions involving cards but do not pertain to the definition of discarding. Moving a card from the battlefield to the graveyard indicates a permanent being destroyed or sacrificed, returning it to the owner's library involves shuffling or placing a card back into their deck, and exiling a card means removing it from play, often with additional effects associated with that action. Each of these actions has specific rules and implications that differ significantly from the process of discarding.