What do biologists mean when they use the term 'unit' to refer to cells?

Answer 1

This is for the purposes of standardization. While varying organisms have uniquely structured cells (i.e plants having chloroplasts and cell walls, animals having mitochondria, and bacteria having plasmids), all fulfill the basic functions of life and having that benchmark is useful for the purposes of comparison. So when a biologist uses the term unit, it is typically in reference to the functional capacity of cells.