[mcbits]

Currently the broadest definition of life would be something like: A self-replicating cellular structure having a metabolism to organize resource input and waste. "Cellular" lets us rule out things like fire and crystals, but viruses and RNA soup are also swept away.

I think if we found a population of some kind of self-replicating RNA analogues on another planet, it would be significant enough to revisit that "cellular" constraint. On the other hand, that definition is already so broad that we could conceivably build self-replicating 3D printers that would qualify as life.

We might also take a top-down approach and come up with a good definition of "ecosystem", and then define living things in terms of their role in an ecosystem.