chaos [LME] A chaos from Greek khaos was originally ‘a gaping void, chasm’. The word later came to refer to the formless matter out of which the universe was thought to have been formed, from which developed the current meaning, ‘utter confusion and disorder’—first used by Shakespeare in Troilus and Cressida. See also gas. In the 1980s scientists pondered the notion that a butterfly fluttering its wings in Rio de Janeiro could start a chain of events that would eventually change the weather in Chicago. They dubbed this the butterfly effect. It is a central idea of chaos theory, a branch of mathematics that deals with complex systems whose behaviour is highly sensitive to slight changes in conditions.