Generally speaking, people say “three-bet” when referring to a reraise that happens before the flop in hold’em. (Occasionally you’ll hear some use the term when talking about post-flop action, though not that often.) In other words, the big blind is the first forced bet, the first raise is a second bet (although that initial raise is never, ever called a “two-bet”), then the first reraise becomes a “three-bet.”
I remember a few years ago reading some discussion about the term, probably inspired by its having first become somewhat popular and a person on a forum wondering about its origin. I recall the explanation for the term connected it to fixed-limit hold’em where the first raise equals two bets, the next equals three, and so on. Even though the raises in no-limit hold’em aren’t of fixed amounts, the terminology was borrowed and used in the same way to describe successive raises/reraises.
I can’t recall the first time I heard the term, but I remember being a little confused by it initially. It’s really not obvious what a “three-bet” means if you’ve never heard the term before, but nowadays almost everyone says “he three-bet” rather than “he reraised” when discussing preflop play. I guess I’ve become conscious of the term’s less-than-obvious meaning thanks to working with Learn.PokerNews and thinking more specifically about new and beginning players who perhaps aren’t up on all of the terminology just yet.
In any event, now everyone uses the term, and in fact it seems almost weird not to. It is so common of a term there’s a clothing line named after it. PokerListings even calls its daily recap of three big stories of the day the “Daily 3-bet.”
I brought the subject up with a friend today and we speculated that the increased use of the term likely coincided with more adventurous preflop betting -- that is, with more and more three-bets actually occurring before the flop. And with all of the “light” reraising and “clicking back” (i.e., raising/reraising the minimum) becoming popular, that has given even more impetus to people using “three-bet,” “four-bet,” and so on when talking about preflop action.
It is helpful, actually, as a kind of shorthand to say “he five-bet shoved” as a way of quickly explaining how many reraises preceded the all-in bet. Or to distinguish between three-betting and four-betting (and five-betting, etc.) when discussing preflop strategy and putting players on ranges and so forth.
Still, it’s a curious term, and one that continues to have a kind of odd disconnect for me. After all, the “three-bet” is the second action (when speaking of preflop betting). Even though I understand the term, there’s a strangeness to it that I’m also always aware of when I hear or read it.