Overpair
A pocket pair higher than every card on the board: aces on K♠ T♦ 4♠. No single card in the opponent's hand beats it, only two together.
An overpair is a pocket pair higher than everything on the board. Aces on K♠ T♦ 4♠, kings on J♣ 8♠ 8♥. One card in the opponent's hand will not beat it; he needs two: a pair with the board, a set, two pair, a straight, a flush. On the flop it is usually the best hand, and it often bets three streets.
The trouble with an overpair is that it does not grow. Flop, turn, river, it is still one pair, while the opponent's range gets stronger with every street he pays on. By the river, against three bets, an overpair beats bluffs and nothing else; the made hands that got here are almost all higher. An overpair loses more money than any other hand because it is the hardest one to stop with.
An example from session 4
A♥A♦ on K♠ T♦ 4♠ T♥; I 3-bet before the flop and bet the flop. On the turn I bet the whole stack, $91.28 into a $91 pot; the opponent folded, +$86. The bet won and was still a mistake: of the 70 combinations he could have arrived with, only 16 could pay a pot-sized bet, and all 16 beat me. A smaller bet would have collected from the hands I beat; the all-in kept only the hands that beat me in the pot.
The same in business
An overpair is a strong starting position that stops being strong while you hold it: market leadership, a patent, an exclusive contract. It does not grow, while the competitors who keep paying to enter do. The most expensive losses come from a strong position you could not leave in time.
The whole hand is in session 4: “Sometimes you name a price and hope nobody takes it”.
The other terms are in the glossary.