I think it stands for a game with high production values, a game with a lot of money and work invested in, and thus should result in a huge seller.
I don't know if there is any true definition though, but I found an article from 1998 by game designer Brad Wardell (don't know who he is...)
http://www.stardock.com/stardock/articles/Trenches/Trenches1198.html
Part of the article:
"In order to understand why they would do this we need to have a primer on how the game market works now.
Presently, in 1998 you have basically three types of games: AAA, A, and B games. You’ll hear about these terms from time to time so let’s define them.
AAA game means games that have almost unlimited budgets and are media events. Blizzard is the AAA game company these days. They won’t release anything that doesn’t fall under AAA. They killed Warcraft Adventures not because it was a bad game but because it would have been only an "A" rated game.
AAA, A, and B level games have nothing to do with how good the game is. If I wrote the world’s greatest space invader’s clone today and even if it had great graphics, great sound, and was totally rock solid, it would still be a B class game. Only a handful of games each year make it out as AAA because the bar is so high to be a AAA game. It costs millions of dollars to create a AAA game. My personal favorite game, Total Annihilation, barely makes it out as a AAA game because it didn’t have full motion video through out, the units didn’t talk. It’s a AAA game still but just barely. So even the best and funnest games may not be AAA games. Starcraft is a great example of a AAA game and I’ll use it because it’s also an excellent game.
Games like Entrepreneur, Panzer General and Warlords III would be great examples of A games. They may be as fun or even funner than AAA games but don’t have the budgets behind them of a Starcraft. No full motion video between every level, they are about the game, not the game and experience.
Deer Hunter is a great example of a B class game in quality. Cheap to make. And where Deer Hunter changed the world was in discovering that a B level game can now make as much money, if not more than a AAA game. And believe me, the game designers of AAA game companies are probably sweating a bit about Deer Hunter. Because corporations are about profit and if they can make more money cranking out B titles they will. But that’s for another discussion entirely.
Historically, AAA, A, and B games represented how much money you’d put in and get back. A AAA game may cost a ton to create but they bring back the big bucks. They are the games of the year, they are the 2 million plus unit sellers. Myst, Dialbo, and Starcraft, these are AAA games."
