This is a term brought up largely because of hole cams. Once it became apparent the camera could see all the cards that have been in play during a hand, mucked or still active, the camera could tell us how many 9's were still in play, or 5's, using the example above.
Live cards thus means that as far as can be told, all of the cards considered live cards are still available and in the deck, not in the muck or your opponents hand.
Which brings up the follow up question;
What do you call it when one of the potential winning cards is known to be in the muck. Good example might be the OESD, where one of the cards is known to be gone. Only 1, what do you call that? semi-live?, Semi-dead? life support? ICU?