GameProducer.net (http://www.gameproducer.net/2006/05/26/what-are-aaa-titles/)'s take on what a 'AAA' game is:

Question:
What are AAA titles?

Short answer:
High-quality games with high budget.

Longer answer:
There is a debate that takes into considering several aspects that AAA titles should have.
Some of the most common qualities that AAA titles should have (according to some experts) are:
- High-quality
- Broad market
- High sales
- Large teams
- Big budget
- Polished audio-visual direction
- Perfect technical and artistic execution
- Playable & fully enjoyable within the first five minutes of play
- Exhaustively tested
- Bug free
- Great usability
- Continous, balanced entertainment from beginning to end
- Great graphical user interface
- First place in the markets, and great marketing
- Hype

Some of the qualities are more arguable than others (like playability in 5 minutes).
Some argue whether AAA titles should have certain budget or certain amount of sales.

In general, AAA refers to a game of the highest quality.

Or does it..?