A game with a budget considerably higher than the average AAA game.
There's no specific numbers, but the ones considered AAAA use to be those with like over maybe 4000-5000 people in the game credits and that took over half a decade to be made. Rare exceptions like GTA6, Fortnite, CoD, Diablo IV, Far Cry 6, Assassin's Creed Shadows, Halo Infinite, etc.
A game like GTA6 can afford a budget of a Billion or so, because they're 100% sure the game will generate way more than that.
We're talking AAAA would have total budgets that would go from maybe around $400M to way above that, when the average AAA released nowadays has a total budget of aprox. around $100-350M. Nowadays AA games would be games with a budget in the range of aprox. $10-80M.
I'm talking about the full budget, including development in the lead game dev studio, plus support and outsourcing studio, plus marketing and PR, plus everything related to publishing. Sometimes you'll find way smaller budget numbers, but that only refer to the development part, or only to the development in the lead dev studio (in AAA games, only around 10% of the people who works in a game is from the lead studio).
Please consider these budget ranges are rough estimates, and that they highly increase (often aprox. double) every new generation.