ghost set -- a set not on the board that is defined by three pairs of cards, such that each pair of cards completes a set with a distinct card in the ghost set.
spirit card -- a card not on the board that completes a set with three pairs of cards on the board.
The game Set can then be played as usual, but players look for the six cards that define ghost sets or spirit cards instead of regular sets. It should be noted that game play is considerably slower because these sets are rarer (I believe. Maybe I'll do the math later.).
I learned about both these variations this weekend. The card above is my spirit card, the first one I found.
Let's play.
