It's the theory that one small effect like a butterfly flapping its wings in the Amazon can cause another effect of slightly greater power. That in turn causes another slightly greater effect and so on until the effect is so great that it is almost unfathomable that it was caused by a butterfly flapping its wings like a tornado or something.

I'm not able to give a tight definition, but it deals with systems which are impossible to predict because variations in the initial conditions too small to be measurable or detectable can drastically affect their behavior. You will sometimes hear the phrase "extreme sensitivity to intital conditions". The butterfly effect is a theoretical example of such sensitivity, and relates to the limits of long-range weather forercasting. It essentally means that in a mathematical model of weather, accuracy greater than random guessing beyond a few days ahead, would require truly infinite precision in the measurement of the initial conditions--that is, the position, speed, and direction of every molecule, photon, butterfly, human, and more, in real time. The theory assumes that what looks chaotic is still deterministic (governed by normal laws of physics), but beyond human capacity for measurement and prediction. Chaos theorists are interested in whether patterns may be found either within or arising from these chaotic systems. Here's a fairly basic online intro: http://order.ph.utexas.edu/chaos/index.html

Others will give you good scientific explanations. Sometimes it's best to just be very simple = For example, here is one visual of Chaos Theory "at work": Look at a whole Boston fern Look at only one individual fern stem and leaf coming off the whole plant. Look at each of the individual smaller leaves on that one larger leaf stem. Notice that each of the little leaves on that one stem look exactly like the larger leaf. And in turn ... look like the whole plant. We cannot predict how the plant will grow but can notice the similarities within it. Chaos. Chaos Theory.

This is an excerpt from the link below (that's why I put it in quotes...) --http://www.imho.com/grae/chaos/chaos.html "The name "chaos theory" comes from the fact that the systems that the theory describes are apparently disordered, but chaos theory is really about finding the underlying order in apparently random data. When was chaos first discovered? The first true experimenter in chaos was a meteorologist, named Edward Lorenz. In 1960, he was working on the problem of weather prediction. He had a computer set up, with a set of twelve equations to model the weather. It didn't predict the weather itself. However this computer program did theoretically predict what the weather might be. One day in 1961, he wanted to see a particular sequence again. To save time, he started in the middle of the sequence, instead of the beginning. He entered the number off his printout and left to let it run. When he came back an hour later, the sequence had evolved differently. Instead of the same pattern as before, it diverged from the pattern, ending up wildly different from the original. (See figure 1.) Eventually he figured out what happened. The computer stored the numbers to six decimal places in its memory. To save paper, he only had it print out three decimal places. In the original sequence, the number was .506127, and he had only typed the first three digits, .506. By all conventional ideas of the time, it should have worked. He should have gotten a sequence very close to the original sequence. A scientist considers himself lucky if he can get measurements with accuracy to three decimal places. Surely the fourth and fifth, impossible to measure using reasonable methods, can't have a huge effect on the outcome of the experiment. Lorenz proved this idea wrong. This effect came to be known as the butterfly effect. The amount of difference in the starting points of the two curves is so small that it is comparable to a butterfly flapping its wings. "The flapping of a single butterfly's wing today produces a tiny change in the state of the atmosphere. Over a period of time, what the atmosphere actually does diverges from what it would have done. So, in a month's time, a tornado that would have devastated the Indonesian coast doesn't happen. Or maybe one that wasn't going to happen, does." (Ian Stewart, Does God Play Dice? The Mathematics of Chaos, pg. 141) This phenomenon, common to chaos theory, is also known as sensitive dependence on initial conditions. Just a small change in the initial conditions can drastically change the long-term behavior of a system. Such a small amount of difference in a measurement might be considered experimental noise, background noise, or an inaccuracy of the equipment. Such things are impossible to avoid in even the most isolated lab. With a starting number of 2, the final result can be entirely different from the same system with a starting value of 2.000001. It is simply impossible to achieve this level of accuracy - just try and measure something to the nearest millionth of an inch! From this idea, Lorenz stated that it is impossible to predict the weather accurately. However, this discovery led Lorenz on to other aspects of what eventually came to be known as chaos theory. " p.s. I'm not a man (directed toward john mitchell)...

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