"Ethnicity" could be defined as "allegded identity".
The formation of an "ethnicity" is rather complex and englobes many factors, among the most important of which are language, religion, history, and ancestry.
Every "ethnic group" alive today may also feel closer or more distant to other ethnic groups, depending on the affinity they share with each other.
In every region, depending on history, the main pillars of ethnicity vary.
In the U.S. and South Africa the main identity is forged on "race".
In Central Asia it is the linguistic family (Turkic, Mongol, Iranian)
In the Middle East and North Africa it is religion.
In Europe it is a combination of factors, including citizenship, language, religion, and wealth.
Personally, I believe that LANGUAGE is the most important factor because it is the more direct. If you can communicate with someone, you naturally share a greater affinity.
We are posting on this forum because we all speak English, right?
If I were posting on a Spanish forum now most of you will have no idea.
One very curious observation is that "wealth" is playing an ever-important role in "ethnic identity" and in "ethnic affinity".
Wealthy countries everywhere in the world live a similar lifestyle whether they are British, French, Japanese, or Canadian.
A middle-class Spaniard in Barcelona might share a great deal of affinity with a middle-class South Korean than with a Bolivian farmer; despite the Spaniard and the Bolivian share the same language and religion.