In Greek mythology, Chaos or Khaos is the primeval state of existence from which the first gods appeared. In Greek it is Χαος, which is usually pronounced similarly to "house", but correctly in ancient Greek as "kh-a-oss"; it means "gaping void", from the verb χαινω "gape, be wide open", Indo-European *"ghen-", *"ghn-"; compare English "chasm" and "yawn", Anglo-Saxon geanian = "to gape".
According to Hesiod's Theogonia, Chaos was the nothingness out of which the first objects of existence appeared. These first beings, described as children of Chaos alone, were Gaia (the Earth), Tartarus (the Underworld), Eros (desire), Nyx (the darkness of the night) and Erebus (the darkness of the Underworld). Thus, at the very start of his story, Hesiod establishes the deities related to each element known to man, beginning with the primordial elements: the Earth, the starry Sky, the Sea.
Theogonia presents two ways to come to life, either by division (Gaia, Nyx), or by mating. After Gaia, almost all the deities brought to life by division are negative concepts (Death, Distress, Sarcasm, Deception, and so on) and for the most part, they are fathered by Nyx. From this point on is set the model for reproduction, from the action of two entities, male and female, as it appears in the divine world in response to human society. So the first answer by the myth to the question "What is the cause of this?" becomes "This is the father and this is the mother".
Furthermore, all deities generated by division almost never become allies with those of male-female lineage.
The original meaning of Χαος was "Space, the great outer void".
Ovid, in his Metamorphoses, described Chaos as "rather a crude and indigested mass, a lifeless lump, unfashioned and unframed, of jarring seeds and justly Chaos named". From that, its meaning evolved into the modern familiar "complete disorder", and the word "Chaos" is used by astronomers in Mars placenames to mean "area of disorderly faulted terrain".
Chaos features three main characteristics:
it is a bottomless gulf where anything falls endlessly: That the Earth will emerge from it to offer a stable ground, radically contrasts with Chaos;
it is a place without any possible orientation, where anything falls in every direction;
it is a space that separates, that divides: after the Earth and the Sky parted, Chaos remains between both.
Ovid, Metamorphoses (I, 7)
Hesiod, Theogony (116; 123-132)