What JJS said.
The $2 blind is the first bet. Callers are calling the 1 bet. If someone raises (say the cut off raises to $6), that's the second bet. Now if the button calls they're "calling 2 bets cold", or "cold calling". If however the button re-raises (to $18 for example), that's the third bet, so now he's 3 betting. If the small blind calls, he would be said to be "calling 3 cold". If the small blind re-raises again however, he'll be putting in the 4th bet.
While this is out there, a term I commonly seen misused is "raise" and "re-raise". I will see players in the cut off's position above make it $6 to go and say "I re-raise". No, you don't re-raise, because there is not a raise in front of you. When the pot is not yet raised and there is only an initial bet (the "1 bet" above), your options are to fold, call, or raise. And then on the flop it's checked to them, and they say "I raise $10″. No, the action is checked to you, your options are to check or bet… you can not "raise" unless there is a bet to be raised. Sorry if this sounds nit picky, but it's kind of a pet peeve of mine, and I hear these terms missused constantly.