A proxy card is an easily-acquired or home-made substitute for a collectible card. A proxy is used when a collectible card game player does not own a card, and it would be impractical for such purposes to acquire the card. This usually occurs when a player desires a card that is cost-prohibitive, or is "playtesting" with many possible cards. When doing intensive training for a competitive tournament, it often makes more sense to use proxy cards while figuring out which cards will be brought to the tournament. Another card is substituted and serves the same function during gameplay as the actual card would.
A proxy can also be used in cases where a player owns a very valuable card, but does not wish to damage it by using it in actual play.