Chaos (derived from the Ancient Greek , Chaos) typically refers to a state lacking order or predictability. In ancient Greece, it referred to the initial state of the universe, and, by extension, space, darkness, or an abyss. In modern English, it is used in classical studies with this original meaning; in mathematics and science to refer to a very specific kind of unpredictability; and informally to mean a state of confusion.

Chaos in mythology, literature, and religion

Rather a rude and indigested mass:
A lifeless lump, unfashion'd, and unfram'd,
Of jarring seeds; and justly Chaos nam'd.
No sun was lighted up, the world to view;
No moon did yet her blunted horns renew:
Nor yet was Earth suspended in the sky,
Nor pois'd, did on her own foundations lye:
Nor seas about the shores their arms had thrown;
But earth, and air, and water, were in one.
Thus air was void of light, and earth unstable,
And water's dark abyss unnavigable.

Scientific and mathematical chaos

Chaotic systems consequently look random. However, they are actually deterministic systems governed by physical or mathematical laws (predictable in principle, if you have exact information) that are impossible to predict in practice beyond a certain point. A commonly used example is weather forecasting, which is only possible up to about a week ahead.

Edward Lorenz and Henri Poincaré were early pioneers of chaos theory, and James Gleick's 1987 book Chaos: Making a New Science helped to popularise the field. A number of philosophers have used the existence of chaos in this sense in arguments about free will.

More recently, computer scientist Christopher Langton in 1990 coined the phrase "edge of chaos" to refer to the behaviour of certain classes of cellular automata. The phrase has since come to refer to a metaphor that some physical, biological, economic, and social systems operate in a region where complexity is maximal, balanced between order, on the one hand, and randomness or chaos, on the other.