From the Greek, Khaos, meaning gaping void, abyss, the sense of the word Chaos is that of senseless, frightening turmoil that will lead to destruction.
Mathematics adopted it in the 1960s and uses it specifically for systems that, though containing no unknown elements, can produce seemingly random outcomes when tiny differences in initial conditions are present.
The most famous invocation of mathematical chaos is The Butterfly Effect. A small input to a system here results in an unpredictable tornado in another part of the world.
Chaotic systems are a type of complex system and though unpredictable, can be understood after the fact. They are also able to produce patterns such as fractals.
In Cynefin, Chaos is used to characterise situations where no sense can be made from the data and no existing practices seem applicable. Then, we are advised to act quickly and be ready to change course as options become clear. This is the domain of the crisis.