In its broadest sense, qi can be thought of as the vibratory nature of reality: how at the atomic level, all of manifest existence is energy – an intelligent, luminous “emptiness” appearing as this form and then that, like waves rising from and then dissolving back into the ocean. Our perception of solidity – of forms as fixed and lasting “things” – is just that: a perception, based upon habitual ways of conceiving of ourselves and our world.

From: taoism.about.com