My oldest just showed me Altitude (a game he just purchased on Steam) and said, "I think this is an indie game.". Me being the annoying thought provoking type father I tend to be said, "Oh really? And what makes a game an indie game?" After talking about it for a bit, my son the always logical black and white type thinker he is said, "This is easy. Indie games have less than 100 people in the credits." So I guess there you have it, Indie has nothing to do with style, has nothing to do with the size of the company, nothing to do with how much money was invested in the game, how creative the game is, whether you used to work for a game company and started your own, nope it's none of those things.
100 people or less in the credits, you're indie. More than that you missed the indie boat. End of discussion I guess?