Often used to refer to a community card that ranks higher than one's pocket pair. For example, if you hold {j-Spades}{j-Diamonds} and the flop comes {q-Diamonds}{9-Clubs}{4-Spades}, the flop has one "overcard" (the queen) to your pair.

Can also be used to refer hole cards that rank higher than any card appearing on the board. For example, if you have {a-Clubs}{k-Spades} and the flop comes {q-Diamonds}{9-Clubs}{4-Spades}, you have no pair but still have two "overcards" to the board.