Anything that can be described as impossible, or defying a set law of the universe; something which has the ability to disrupt the space time continuum.
In Back To The Future Part III, as the third log hits in the locomative, the delorean's front lifts up into the air, which is impossible with the set limits shown in the movie. Third Log Syndrome.
by KevinTehLeet September 18, 2006