Life is very useful as a term because it allows you to define a 'not living' term as well: dead. And it has meaning at the highest level. But if you start looking at things in a more detailed way even death doesn't arrive 'all at once' for multi cellular organisms, for instance a dead person's hair still grows to the point that corpses need to be shaved. And a virus may be dead by one persons view on what 'life' is all about but alive by someone else's definition. And depending on the context both of them may be right.

The definition is fuzzy because the concept is fuzzy! Even something that we in every day life see as settled such as a species is not always clear-cut. Cat or dog? That's usually easy. Member of a species yes or no? Not so easy, and in some cases subject to considerable debate and even then unresolved.

For instance:

https://en.wikipedia.org/wiki/Species_complex

And:

https://en.wikipedia.org/wiki/Ring_species

Boundaries are hard, just like naming things.