A AAA game, or pronounced "triple-A game", is generally a title developed by a large studio, funded by a massive budget.
These games will have a marketing budget in the multiple-millions of dollars, and are planned to earn out in excess of one million titles sold. Investors/publishers expect a multiple-of-cost return on their investment. In order to recoup general development costs, publishers will generally produce the title for the major platforms (currently Xbox 360, PS3, and PC) to maximize profits, unless it is a console exclusive, in which case the console maker will pay for exclusivity to offset the loss of potential profit to the developer.
Pronunciation: triple-A game