I am not sure if that can be actually defined. Have you a definition?
Good question. I am reciting what I have heard over the decades that Jewish ethnicity is passed down matrilineally from Sarah, Abrahma's wife. I presume there are genetic markers peculiar to the different human tribes. Of course ethnicity does not dictate one's religious beliefs.
In the U.S., American ethnic Jews seem to be roughly divided as below ...
20% atheistic
20% agnostic/ non-practicing
40% Reformed Judaism tradition
20% Conservative/ Orthodox traditions