Beyond that definition nearly lifted directly from the text, the usage of the word chaos in the context of the reading reminded me a lot of the interpretations of stories of genesis/creation, relevant to chaos, that I was exposed to in CEUS-320 (Mongolian Buddhist Mathematics/Astrology) last spring. In that perspective, chaos is thought to be the essential state of things, of reality. Myths or stories of creation then act as metaphorical representations of man's ascent to cognizance and self-awareness, to the point where the phenomena that surrounds him is ordered and classified based on apparently universal perceptions.
The creation that Ovid writes of is but one of many examples of this idea. Others include God's slaying of Leviathan in the Bible, the Babylonian god of order Marduk's dispatching of the dragon of Chaos, Tiamat, or even the modern theory of evolution by natural selection, where humanity gradually evolved from a primordial animal state into one of observation, manipulation, and rationality.
Reading the text in this context casts many lines in a light different from their literal meaning. Lines 6-10 read: "Before the seas and lands had been created, / before the sky that covers everything, / Nature displayed a single aspect only / throughout the cosmos; Chaos was its name / a shapeless unwrought mass of inert bulk." Assuming that Ovid is speaking along the lines of the metaphor described above, he does not mean that the seas and lands literally did not exist, but rather, were all part of the formless and meaningless void that is the essential state of reality. Chaos is then the representation of the unbreakable union of all that is.
It was only after man's ascent into self-awareness, a journey spurred by any number of possible sources i.e. god, natural selection etc., that the concepts which are commonly used to identify phenomena regarded as "sea" and "land" began to exist. It was the advent of these sorts of concepts, or reference points, that allowed mankind to navigate its way through a chaotic and formless reality, designating meaning and form, and inventing the previously non-existent notions of individual objects and ideas. It was from this starting point, the beginning of man's ordering of chaos, the advent of organization and individuality, that the four ages Ovid describes could follow, leading to the state of the world and reality as we know it today.