Much of science is about defining words in ways that match the underlying general structure of the system being studied. A subset of scientists want to come up

Much of science is about defining words in ways that match the underlying general structure of the system being studied. A subset of scientists want to come up with an operational definition of "What is life", which may or may not include things like viruses and mitochondria. As you say, it's mostly definitional, but by defining this, we can potentially make our understanding match up with the latent reality.