There's quite a few different kind of flops.

1. Universal flop. This is something that failed on every level, critics hate it, fans hate it and its sales were insufficient to make a profit.

2. Critical flop. A game that failed to impress the critics and most likely a majority of the fans. It could still be a commercial success if it had a lot of hype, good marketing or a solid fan-base.

3. Commercial flop. Most critics and consumers could agree that it's a good/great game yet it still couldn't sell well enough to make a profit.

4. Relative flop. A game that despite a good critical reception and selling enough to be profitable, falls significantly below what past games in the series have achieved on average.

Flop is a product that fails to turn a profit. Though in the world of videogames critical reception can make a big difference aswell.

It didn't meet expectations

Damn you, I was going to say that!

Except I wasn't kidding... Muahaha.

Whatch out... we got a badass over here!

Sonic The Hedgehog (2006) was a flop but it still sold super well. I don't consider a game a flop if it doesn't sell well. A game can also be unsuccessful is delivering quality gameplay etc.