Everything in every direction, all the time. Unbridled, radiant energy.

Incidentally, although the octo-form cross is an ancient symbol, usually associated with the solar wheel, most people believe the association with Chaos to be the modern invention of "Elric" author Michael Moorcock. It passed into popular usage as a result of being appropriated from his novels into role playing games.

"Moorcock says:

The origin of the Chaos Symbol was me doodling sitting at the kitchen table and wondering what to tell Jim Cawthorn the arms of Chaos looked like. I drew a straightforward geographical quadrant (which often has arrows, too!) – N, S, E, W – and then added another four directions and that was that – eight arrows representing all possibilities, one arrow representing the single, certain road of Law. I have since been told to my face that it is an "ancient symbol of Chaos" and if it is then it confirms a lot of theories about the race mind. … The symbol, drawn by Jim Cawthorn, first appeared on an Elric cover of Science Fantasy in 1962, then later appeared in his first comic version of Stormbringer done by Savoy [ISBN 0704502267]. "

from Wikipedia, en.wikipedia.org/wiki/Symbol_of_Chaos