What does the word "flop" mean?

[Tizona]

It seems like many forum users use flop as a synonym for "not a huge hit" which I find a little silly. As if there is no grey area between massive hit and failure. To me the only thing I'll call a "flop" is a game that has very high expectations, that doesn't live up to those expectations by a wide and surprising margin. A game expected to sell 2 million only selling 1.6 is not flop. A game expected to sell 2 million and only selling 500K would be more of a flop.