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Rather than talk about what consciousness is, I'll tell you there are two necessary things for consciousness to arise:

wakefulness and awareness.

It goes like this:

wakefulness --> awareness --> consciousness

Which means that you need to be awake to be aware and you need to be aware in order to be conscious.

If someone has a disorder affecting wakefulness, such as being in a coma, then he/she can't be aware and therefore cannot be conscious. If someone is awake but is in a vegetative state, he/she can't have awareness and therefore cannot be conscious either.