Name a card type.
Each other player discards the top card of his deck:
If it is the named type, +1 Card;
If it is not the named type, discard a card.
Clearly name one of the card types to make this choice known to the other players. It does not necessarily have to be a card type in the Supply, but does have to be a type defined in the rules (e.g. "Treasure", "Action", "Attack", etc. "Cards starting with the letter S" is not a legitimate choice).
Each player discards a card from the top of their deck, and if it is the type you named, you get +1 Card. If not, you must discard a card. Since order is important here, start with the player on your left and progress clockwise around the table.
If a card is discarded which has more than one type, all that matters is whether at least one of the types matches the type you named.