3-bet
A raise over someone else's raise: the blind is the first bet, the open is the second, the re-raise is the third. Hence the name.
A 3-bet is a raise in answer to a raise. Someone opened to $2.50, you raise to $8. The count starts from the blind: it is the first bet, the open the second, your re-raise the third. The answer to it is a 4-bet, and so on. Each step is three or four times the last, which is why after a 4-bet at NL100 there is almost no money left on the table.
It is not only aces that 3-bet. The 3-betting range has two parts: hands that want a big pot because they are ahead, and hands that want a fold now because they have odds against an opening range but none against the hands that answer a 3-bet. A player who 3-bets only strong hands hands the opponent free information with every re-raise.
An example from session 2
K♠K♣. The opponent opens to $2.50, I 3-bet to $8, he 4-bets to $21.13, I move all in for $107.71. He folds, +$19. The fold is not the valuable part: even against the narrowest range that answers such a bet, kings are ahead 57%. Whatever he did, the decision stays positive by $12.63.
The same in business
A 3-bet is a counter-offer that changes the scale of the deal, not just the price. Answering an offer with a counter three times larger tests not the other side's willingness to pay but its willingness to go on at all. Whoever counters only when he is sure he wants the deal announces it with every counter.
The whole hand is in session 2: “What strengthened everyone strengthened no one”.
The other terms are in the glossary.