I think the easiest definition for life, as we know it on our our planet, is to take Dawkins' perspective from 'The Selfish Gene'.

Life is self-replicant genes and the various machines they build around themselves.

No more. No less.

Bacteria are the quintessential gene replication machine. Viruses make complete sense here, hijacking the machines of the bacteria to replicate their genes. The virus is definitely alive. It replicates just fine. It just uses machinery in its environment, found in other cells, to do this, rather than packing it around itself. If life is the genes, there's no paradox at all. A hitchhiker is no less a traveler for the lack of their own car.

Of course, when we think of life, we think of the meta-structures we see, multicellular coalitions of cells that specialize and depend on one another. And that's fine. It's what we interact with and what is important in our day to day existence.

The human body has something like 100 trillion bacteria in its gut, with only 10 trillion cells cooperating to be the human.

From worms to fish, to frogs, to lizards, rats, monkeys and humans, we are essentially a protective tube around the payload of cells found in the gut biome we carry around inside us, just as those cells are protective bubbles around the genes inside them.

Scientists continue to be surprised at the effects these multitudinous populations within us have upon us. But it's not really that surprising if you consider things from their point of view. We're spaceships, carrying them around in an artificial environment, purpose grown for them to thrive in. They send out chemical hormones into our blood stream to signal needs, and they and we have been shaped together over time, and to depend on one another. So we respond in a way similar to what was successful with prior generations of genes and the organisms that contain them.

Does that mean they control us? Obviously not in any intelligent sense. They do add to our wants and needs, creating part of our experience of being. Relatedly, fecal transplants have been used to quell intestinal issues, but too have they been looked at in regards to depression and other mental issues.

What then are we, in this cacophony that genetic replication expresses?

We are gestalts.

Our brains are certainly formed by the genes inside us, but those genes are found in many, and do not encode our experiences, nor our memories, nor our decisions on what wants and needs to value and follow, regardless of which feel better in any immediate sense. Genes have no art, nor science to them. They are mindless, yet from them we are. Minds born of untold trillions of that live mindlessly.

I find Dawkins' further considerations on the ideas of the 'meme', being a replicator that exists only as a pattern in our memories and communications, to be fantastic here, and possibly a perfect way of generalizing life itself.

Much of what we call our self is a selection and rejection by preference of ideas we are exposed to from other people. In our genes, we do not find our languages, nor the metaphors with which they express our experiences, designs and desires between each other. We do not find our sports, nor rules, nor borders, nor a hundred other things we consider daily. These things come from without. From other people. We transmit them to each other, and to our children, and they to theirs. These concepts are within us, perhaps define what we see as 'us', but are not any physical sense part of us. Society lives in our collective minds, encoded in ideas, that exist in brains that are collectives of cells all of which contain genes, genes which, alone in all this tumult, replicate nearly flawlessly with every generation. The containers change around them, but all life shares these uncountable perfect little clones within us.

Perhaps life will one day be generalized as any self-similar replicating pattern in space and time, with various plateaus and groups of life for our joyful categorization to file examples into.

But for now, in our situation, genes themselves remain the best and most accurate definition of life that I can imagine