But I'm not a rapper.

I think it should be up to the publishers what was a flop and what not because they are the only ones who can and should measure success. We can only guess and hope that our favorite games weren't flops in the eye of the publisher.

When it doesn't make the projected profits. Anyone remember the movie After Earth? it cost 130million to make and made 240m, but was called a flop because it didn't meet the numbers it was expected.

A game is a flop when it sells less than Knack ( AKA the hottest game on the market yo).

Small franchise/new IP sells less than 1 million is a flop.
A middle of the road franchise sells less than 2 million copies is a flop.
A top of the range franchise sells less than 5 million per game is a flop.
A sales flop depends on the popularity of the franchise. Games these days cost a lot to develop, market and mass produce. A higher number needs to be sold by big franchises to make a profit.

When it fails to break even on its costs and causes the dev/publisher to lose money.

Fails to meet [publisher / HW seller] expectations by >50%.

That can be used in 2 ways: expectation to move consoles (as in system selling exclusives); and expectation for game sales.

Taking DKTF haredware boost across the 2 weeks of launch it lifted Wii U by 14% + 10%. A genuine system seller should cause a >50% boost, and early generation it should cause a 100%+ boost. But was DKTF expected to be a system seller? I don't know. I also don't know what Nintendo's expectations are for game sales. I presume they would expect it to sell > 1million. So flop would be

When it doesn't sell well and/or it isn't profitable.

If I dont like it, its a flop, I couldnt care less if a company spent 10 years and went bankrupt making it.

Nvm.   I thought the topic was something else, maybe I clicked on the wrong one.