What is Growth and how can we define it to be a defining property of life?
If you are just asking what does growth mean in the context of living systems then it almost always means replication of a unit: cell or genetic material (cell division, DNA replication etc).

Note that increase in cell volume or the inter-cellular space (accretionary growth) also happen in living systems and they are also considered growth. However, at a gross level these processes are indistinguishable from what is happening to wood that is dipped in water.

So growing is not an exclusive feature of living systems but still one of the defining features.