In Dominion, when it says reveal a card, who must I reveal it to?
Reveal means to show their hand to the table.

From the rules, page 7.


  Reveal – when a player reveals a card, he shows a card to all players and
  then returns it to wherever it came from (unless instructed specifically to
  put it elsewhere). If the player is required to reveal cards from the top of
  his Deck, and he does not have enough cards, he shuffles in order to
  reveal the required number of cards.