I've always liked Edith Hamilton's translation of Chaos:
“First there was chaos, the vast immeasurable abyss, outrageous as a sea, dark, wasteful, and wild... From the formless confusion of Chaos, brooded over by unbroken darkness came three children; into this shapeless nothing they were thrown. Erebus, which is the unfathomable depth where death dwells and his two sisters, Nyx, or night, and Gaea, the earth. In the whole universe there was nothing else; all was black, empty, silent, endless."
Chaos is the initial state of things, how they exist pre-thought. What they look like when they're off-stage. Chaos is the state of the unknowable future.