The so-called "butterfly effect" is when some trivial change causes larger repercussions further down the road, leading to a major divergence from what would have happened otherwise. Essentially "unpredictable".
I believe it dated from a (1950's?) sci-fi short story about some guy who pays to "hunt dinosaurs", but he's warned that straying from the clearly defined path or shooting anything other than the specific dinosaur that he paid to shoot (which is destined to die anyway) may result in unpredictable changes. He steps off the path to get a clear shot, shoots his target, and then steps back into the time machine, except that all the letters have changed to French. When he returns to a very different world than the one he left, he finds a crushed butterfly on the bottom of his boot, which caused the disruption of the food chain and 40 million years of slightly divergent history. The term was used as the title of a more recent movie, playing on that effect.
In the cases mentioned in previous posts, there were cumulative effects of multiple events, so even a minor change in one of those events could have caused a significant shift in those later events, many of which affected more than one country. The change to the one event makes the whole following sequence or multiple sequences of events unpredictable. A change that affected one country in WWI, which in turn had a significant effect on several countries in the war, leading to an armistice which then may or may not resemble the one which was signed historically, would have had totally unpredictable consequences in the politics and military developments leading up to WWII.
Butterflies, lots of them. Remember, though, that some species of male butterflies are know to travel up to a mile to tear the limbs and wings off of a competitor. They may be pretty, but they're not all peaceful.
