1. The theory that everything that happens must happen; that you can't save one thing without destroying something else. Coming from the idea of a butterfly caught in a spider's web. You can't save the butterfly without killing the spider (through violence or starvation).
(Also personified in the more popular saying, "Live and Let Die")
2. ... Also the name of a movie that portrays the previous definition. Er, I think. I haven't actually seen it. Is it any good? Excuse me if I don't hold my breath...
Stopping the bombing of Hiroshima would most likely have the butterfly effect that would have costed a multitude more lives.
(Please excuse the politically incorrect referance. My most heart-felt apologies go out to anyone particularly connected the the incident portrayed in this example.)