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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Chaos (derived from the Ancient Greek Χάος, Chaos) typically refers to unpredictability, and is the antithesis of cosmos. The word χάος did not mean "disorder" in classical-period ancient Greece. It meant "the primal emptiness, space" (see Chaos (mythology)). Chaos is derived from the Proto-Indo-Euopean root ghn or ghen meaning "gape, be wide open": compare "chasm" (from Ancient Greek χάσμα, a cleft, slit or gap), and Anglo-Saxon gānian ("yawn"), geanian, ginian ("gape wide"); see also Old Norse Ginnungagap. Due to people misunderstanding early Christian uses of the word, the meaning of the word changed to "disorder". (The Ancient Greek for "disorder" is ταραχή.).
Chaotic systems are systems that look random but aren't. They are actually deterministic systems (predictable if you have enough information) governed by physical laws, that are very difficult to predict accurately (a commonly used example is weather forecasting).
Furthermore, the word gas is probably an alteration of chaos. Particles in gases exhibit chaotic motion, although this was unknown to Jan Baptist van Helmont, the inventor of the term. He is instead believed to have been influenced by the concept of chaos in the occult theories of Paracelsus.