I think that unit is relative concept, which is different in diverse definition. For example, in biological boundary, there are kingdom, phylum, class, order, family, genus and species. Each smaller than the former one, that you can say like that species is a unit of genus. And organism, also means individual, is an unit of species. Cell is an unit of organism and molecule is the unit of organism.
So, what I mean is the unit is not independent, it must have a definable range.
Good question. Hope it would help, just a kind of thinking from a student whose major is biological science.