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late 14c., "gaping void," from Old French chaos (14c.) or directly from Latin chaos, from Greek khaos "abyss, that which gapes wide open, is vast and empty," from *khnwos, from PIE root *gheu- "to gape, yawn" (cf. Greek khaino "I yawn," Old English ginian, Old Norse ginnunga-gap; see yawn (v.)).
Meaning "utter confusion" (c.1600) is extended from theological use of chaos for "the void at the beginning of creation" in Vulgate version of Genesis (1530s in English). The Greek for "disorder" was tarakhe, however the use of chaos here was rooted in Hesiod ("Theogony"), who describes khaos as the primeval emptiness of the Universe, begetter of Erebus and Nyx ("Night"), and in Ovid ("Metamorphoses"), who opposes Khaos to Kosmos, "the ordered Universe." Meaning "orderless confusion" in human affairs is from c.1600. Chaos theory in the modern mathematical sense is attested from c.1977.
The behavior of systems that follow deterministic laws but appear random and unpredictable. Chaotic systems very are sensitive to initial conditions; small changes in those conditions can lead to quite different outcomes. One example of chaotic behavior is the flow of air in conditions of turbulence. See more at fractal.