chaos theory, in mathematics, physics, and other fields, a set of ideas that attempts to reveal structure in aperiodic, unpredictable dynamic systems such as cloud formation or the fluctuation of biological populations. Although chaotic systems obey certain rules that can be described by mathematical equations, chaos theory shows the difficulty of predicting their long-range behavior. In the last half of the 20th cent., theorists in various scientific disciplines began to believe that the type of linear analysis used in classical applied mathematics presumes an orderly periodicity that rarely occurs in nature; in the quest to discover regularities, disorder had been ignored. Thus, chaos theorists have set about constructing deterministic, nonlinear dynamic models that elucidate irregular, unpredictable behavior (see nonlinear dynamics). Some of the early investigators of chaos were the American physicist Mitchell Feigenbaum; the Polish-born mathematician and inventor of fractals (see fractal geometry) Benoit Mandelbrot; the American mathematician James Yorke, who popularized the term "chaos"; and the American meteorologist Edward Lorenz.

Chaos, in Greek religion and mythology, vacant, unfathomable space. From it arose all things, earthly and divine. There are various legends explaining it. In one version, Eurynome rose out of Chaos and created all things. In another, Gaea sprang from Chaos and was the mother of all things. Eventually the word chaos came to mean a great confusion of matter out of which a supreme being created all life.

Behaviour in a complex system that appears irregular or unpredictable but is actually determinate. The apparently random or unpredictable behaviour in systems governed by complicated (nonlinear) deterministic laws is the result of high sensitivity to initial conditions. For example, Edward Lorenz discovered that a simple model of heat convection exhibits chaotic behaviour. In a now-classic example of such sensitivity to initial conditions, he suggested that the mere flapping of a butterfly's wings could eventually result in large-scale changes in the weather (the “butterfly effect”).

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In Greek cosmology, either the primeval emptiness before things came into being or the abyss of Tartarus, the underworld. In Hesiod's Theogony, there was first Chaos, then Gaea and Eros. The offspring of Chaos were Erebus (Darkness) and Nyx (Night). Ovid gave Chaos its modern meaning: the original formless and disordered mass from which the ordered universe is created. The early church fathers applied this interpretation to the creation story in Genesis.

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