Consciousness is the only reason we know reality exists at all. It is the frame for our own subjective experience of reality. As such, the only way we can define it is in terms of subjectivity itself – we must, in effect, mentally point to our own experiences and associate the word with those experiences. The technical name for this is a "private ostensive definition". This is not an orthodox definition, but it establishes what the word is supposed to mean. I must stress that Wittgenstein's private language argument does not apply in this case, because I am not trying to define a private language; I am merely establishing the meaning of the word “consciousness” as used. We can take this a bit further, because it is necessary to ensure that we avoid solipsism (the belief that nothing exists outside our own mind). Within our own consciousness, we are aware of a large number of other beings which behave as if they too are conscious – not just other humans but also most animals, right down to the level of something like an insect or a worm (although precisely how and where we draw the line is very much an open question at this point). If we assume these other beings are actually conscious too, then solipsism can be dismissed.
Also see:
The Hard Problem of Consciousness | Two-Phase Cosmology
The Incoherence of Physicalism | Two-Phase Cosmology
The Evolution of Consciousness | Two-Phase Cosmology
The storm metaphor for consciousness | Two-Phase Cosmology