A card(s) higher than another. If both your cards are higher than any on the board, you hold two over cards. If the board has a card higher than your two, there is an over card showing. Pairing the board is not so valuable if there is an over card that someone else likely has.
A card higher than any visible card.
A card higher than any card on the board. For instance, if you have A Q and the flop comes J-7-3, you don't have a pair, but you have two over cards.