flop /flɒp/

informal to fail; be unsuccessful: the scheme flopped

So it means "a failure", but do you consider, let's say, 3D World a flop because it didn't move systems? Or Gran Turismo 6 because it sold way worse than the last game on the franchise? Like Battlefield 4 because it was a huge dissapointment? Or because it just sells bad, like every Wii U/Vita game TW101, Rayman Legends, or Puppeteer? 

I would consider something a flop if it didn't match the studios own set expectations for it. They were the ones to make the game, so only they could know how much they need to sell/what goals they want to accomplish to consider a project successful.

When a game makes no or barely any money. Because that was just a waste of time and resources for the developer and publisher.
Unfair to call something a flop if it made money.

A flop loses money or makes so little that the entire endeavor was a waste of time.

The only other definition I would consider is if a game is so bad that it brings down a franchise.

Personally, I'm a bit tired of seeing "flop" used seriously to describe something that made money, even if it didn't make as much as projected, or for a game with modest expectations.

When it sells much lower than expectations, or sells less than half of its predecessor.

When publishers and developers just can't make money with the game, simple as that. The Wonderful 101 was effectively a flop, but sometimes it seemed that Nintendo didn't even want to sell the game at all. They just promoted the game on Nintendo Directs, so no worries why this awesome game has those abismal sales.

If it doesn't get a sequel. Honestly that's the only way to tell with 2m+ selling games not being profitable while a game like INfamous Second Son gets all the funds and marketing it needs with less than 2m sales.

no profit

If the came does not sell enough to cover the cost of development, distribution and advertising, it is a flop, If it does, it is not.

That's pretty much all there is to it.

Depends of the brand behind the game.

Halo or Cod < 1 million = HUGE FLOP
 Some unknow JRPG > 100k = SUCESS