A pocket pair above, or below, every card on the board.
An overpair is a strong value hand; an underpair is only good enough to win at showdown. Example. On K♠ 8♦ 3♣, AA is an overpair, 22 is an underpair, and TT, between the top two board cards, is a thin-value hand. 55, between the second card and the bottom one, isn't called an underpair but plays the same as 22.