Chaos derives from the Greek Χαος and typically refers to unpredictability. In the metaphysical sense, it is the opposite of Law and Order: unrestrictive, both creative and destructive.
The word χαος did not mean "disorder" in classical-period Ancient Greece. It meant "the primal emptiness, Space". It is derived f rom the Indo-European root ghn or ghen meaning "gape, be wide open": compare "chasm" (from Greek), and Anglo-Saxon gānian (= "yawn"), geanian, ginian (= "gape wide"); see also Old Norse Ginnunga gap. Due to people misunderstanding early "Christian" uses of the word, the meaning of the word changed to "disorder". (The Ancient Greek for "disorder" is ταραχη.). Mathematically Chaos means an aperiodic deterministic behavior which are very sensitive to its initial conditions see Chaos theory.
An oft-made assumption is that the metaphysical Chaos is Evil and that Law is Good, however it should be noted these are different concepts, and it is quite possible for Chaos to be Good and Law be Evil. One example of this would be the story of Robin Hood, or the real-life world examples of oppressive governments and dictatorships being thrown off by freedom fighters.
Chaos is in the bible also used to refer to the earliest conditions of the universe, the unbounded space and formless matter that existed before the creation of the universe.
Chaos theory, a branch of mathematics and physics.
In astronomy, Chaos is the name of trans-neptunian asteroid (19521) Chaos, which is of the cubewano type.
In geology, a Chaos is an area of broken terrain, often in placenames on Mars.
In biology, Chaos is a type of giant amoeba.
XaoS, a GPL program for real-time zooming in on fractals.