My definition of a bad movie is, well, a bad movie. Except that it can't be one you can just stop watching. It has to be a movie that compels you, somehow, to watch it until the end.
The best example I know of is Van Helsing. Kate Beckinsale stops laughing at the script long enough to run around in heels and play the foil to Wolverine. Enough eye candy and dazzling sets that it's impossible to turn away from this trainwreck.