I’d like to define ethnicity in a very simple way
Not a task I think I’ll succeed at, just something I think is necessary
Ethnicity at it’s most basic level can be broke into 2 parts:
Ancestry being who our parents were. Usually where they are from geographically is an important part of that.
Culture being traditions, norms, customs, mythology, and so on. These are passed to us by our parents, usually passed down from their families over multiple generations.
One bit of nuance here; we all have a tendency to intermingle our ancestry with our culture. We believe that the cultural norms and so on are inherent aspects of our ancestry and biology, which is not true (if you want proof go to a library, a lot of thorough research has been done on the subject).
Often this intermingling turns into a form of ethnocentrism. My ancestors are better than yours. Because my culture is better than yours. And how do I know that!? Because my ancestors say so!
So there you have it
My attempt at a simple explanation
Ethnicity = Ancestry and Culture
The biology of where we come from
And the myths we tell ourselves about who we are. Often those myths tell us what our biology and ancestry means, and often what those myths tell us about our biology is inaccurate