How would you define a flop?

[Reasonable]

For me flop is relative to the game and how well it achieves certain metrics versus reasonable expectations for its sales/critical/market reception

So for example I'd judge something like Flower very differently from Killzone 2 and wouldn't use the same measurement all the time (i.e. 1M sales to be a hit).

I think the true flops tend to be obvious - titles that looked promising, review poorly, get poor online notices, sell poorly vs similar titles and general stink up the gaming universe.

I do notice that since the 360/PS3 fight started (and Wii went in a different direction) lots of people are falling over to claim 'flop' with the most tenuous of justification.

So if it makes money, reviews okay and finds an audience its not a flop IMHO.