Chaos is the formless matter that is said to precede our universe, similar to a Protoverse. This idea originates from Greek mythology but has also been used within a Biblical context.
Chaos can also be described as formless matter in general, which precedes something before it comes into existence. This may not be true for all things, however.
Chaos is something uncertain and something without control or order, it generates improbabilities and inconsistencies, because there is no order to events.