I think there's a clear distinction between sales and critical acclaim/expectations - but simply put -
1) final product < critical expectations-response/personal expectations = disappointment.
2) If a terrible game/movie makes a lot of money - no one calls it a flop - it's successful, but disappointing.
Simply put - even if disappointing - no one calls a sequl that makes more money than its predecessor - a flop. If it's disappointing - and it fails at market - flop is correct - but flop is a strictly commercial term. You've surely all heard of something "critically acclaimed but flopping at the box office" - and a "critical and commercial flop" - but never " terrible flop movie that made 1000 times it's budget back". No - it's just "terrible disappointment that went gangbusters at the box office".
Even if a movie is critically acclaimed - if it makes no money - it's still called a flop. Artistically unfair sure - but as much as I love Arrested Development - it wasn't it's lack of critical acclaim that caused them to cancel it after 3 seasons. (awaits movies/prequel miniseries)
There's a critical "flop" - there's a commercial "flop" - but if it succeeds commercially but not critically - it's not a flop.
TLDR version -
Commercial flop > Critical Flop
Twilight is terrible, and most universally agree - but no one calls it a flop.
Science!
