Same with something like conscious. We do not understand it well enough for any definition to fit.
I think that a good definition for consciousness is the subjective experience of it feeling like something to be. It's the fact that there's qualia to existence. I don't see a reason for why we would need to solve the hard problem of consciousness in order to define a term we use to describe it. As far as I know we don't understand gravity either but everyone still understands that term.
No, I don't think consciousness is necessary for intelligence either. I just couldn't help but comment on that because as a subject it's close to my heart.