The word "chaos" is familiar in everyday speech. It normally means a lack of order or predictability. Thus one says that the weather is chaotic, or that rising particles of smoke are chaotic, or that the stock market is chaotic. It is a lack of predictability that lies behind the mathematical notion of chaos. Both sensitive dependence on initial conditions and the Lyapunov exponent qualify as measures of unpredictability. Thus we have the following definition of chaos.

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