I was asked this on another blog, and wrote a long response. I thought readers of AMNAP might find it of interest:
Let's leave behind conceptual ideas of matter and brains for a second, and go back to first principles.
There are thoughts, sensations, and emotions. This is the raw material for the contents of consciousness. Everything we know can be reduced to one of these basic contents of consciousness. Even materialism itself is a conceptual thought, grouped with a series of other conceptual ideas (atoms, the universe, me, my brain, etc.) Closely associated with thoughts are emotional responses to those thoughts. And the raw material for all these contents are sensations -- light and color, touch, smell, taste and hearing. For the most part, thought is composed of the memory of particular sounds (unless you happen to be deaf) -- even when you read, the words are mentally "heard" to a degree.
So, amazingly enough, even hard-core, reductionistic materialism, is ultimately mind-stuff, just symbols being "heard" within consciousness.
And consciousness is the "space" in which all these thoughts, sensations, and emotions are witnessed. Consciousness (or in other words, pure subjectivity) is all we know, and all we can ever know. The idea of an objective universe (whether it is true or not, let us not concern ourselves with that detail for now) is itself that "squishy" thought-stuff, being witnessed by consciousness.
So the best definition for consciousness that I have is the present moment / present space in which all of the different phenomena are perceived to be unfolding. It cannot be understood by thoughts (because thoughts, after all, are objects which are perceived by consciousness). A thought itself is simply a series of words, which ultimately are simply a stream of sounds pointing at abstractions of reality ("entities").
But always remember what a thought is, and how limited it must therefore be. At best, thoughts can only point at the nature of reality, and experiencing life directly is a more real way to go.