Chaos is a state of extreme confusion and disorder. Putting a dozen dogs and a dozen cats in the same room would probably lead to utter chaos.
The word chaos derives from a Greek word meaning "chasm" or "void," which makes sense, given that chaos also refers to the formless state of matter before the cosmos was created. In math and science, chaos describes a system that will develop in wildly different ways with only tiny changes to the initial conditions. "Controlled chaos" is a phrase often used casually to describe something that looks out of control but which functions according to unseen rules or organization.