1a : a state of utter confusion 

• the blackout caused chaos throughout the city

b : a confused mass or mixture 

• a chaos of television antennas

2 often capitalized : a state of things in which chance is supreme; especially : the confused unorganized state of primordial matter before the creation of distinct forms — compare cosmos

b : the inherent unpredictability in the behavior of a complex natural system (such as the atmosphere, boiling water, or the beating heart)

obsolete : chasm, abyss