A flop is a combo of the two...short run and very little investment back. If the show ran for 5 years, but for some reason didn't make back it's investment (let's say 90%), I wouldn't call it a flop. However, if it ran for two months and it made only 45% of it's investment, then yes, it's a flop.
What if a show ran for 2 months, but it made back 105% investment? I don't think that would be a flop...most likely it would've been a limited engagement then.
"We like to snark around here. Sometimes we actually talk about theater...but we try not to let that get in our way." - dramamama611