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.).
Chaos is the complexity of causality or the relationship between events. This means that any 'seemingly' insignificant event in the universe has the potential to trigger a chain reaction that will change the whole system. A well known saying in connection with this issue is "A butterfly flapping its wings in one part of the world can cause a hurricane on the other side of the earth." This is also known as the "butterfly effect".