How would you even define such a thing?

My grandmother did our family tree and there are all manner of immigrants on it coming to the US at various times over the last 250 years, with some as recently as 80 years ago.

My wife's family can trace back to one of the guys in one of the first Spanish expeditions up the Rio Grande about 400 years back with a smattering of Native American thrown in over the decades for good measure.

What generation of American are our two children? I couldn't even pretend to have any kind of answer for that.

To make things even less clear, my sister-in-law is from Italy and only moved to the US 7 years ago. What generation of immigrant is my nephew?

I'm not trying to just be contentious. I don't even know what the question really means.

Edit: And I forgot about my wife's sister' son. His father is full blooded Navajo. Where does that kid fit in?

To put it into a form that hopefully is proper for this sub, is 'generation of immigrant' something with a defined meaning in an historical context? Was this something other cultures concerned themselves with? Or is this all more of anthropology question?