The fact that something for which, unlike phlogiston or the Aether, we have direct incontrovertible evidence (denial of which evidence puts one on pain of having to justify one's own sanity and fitness for discourse), has yet so eluded definition in strict functional / scientific terms, points more to the likelihood we're suffering from some collective conceptual confusion, afaics.
Rather than consciousness being a primitive pre-scientific intellectual construct, perhaps our current scientific / functional ontology will later be regarded as pre-X, where X is the name for whatever resolution of the aforementioned conceptual confusion we achieve such that it allows us to integrate the fact of consciousness into a unified rational account of the universe.