Ethnicity is a group identity. Unlike nationality and religion which have clear criteria, ethnicity is very subjective. A new ethnic group can be formed from a kingdom/empire, such as Han (From Han Dynasty), English (From Kingdom of England), Uyghur (From Uyghur Khaganate). In most cases ethnic identity is born from tribes. Ethnic identity itself can be born from religion. Jews, for example, are ethno-religious, which means a Jew can either be someone who practice the religion of Judaism or one who descended from ethnic Jews. If one chooses to convert to Judaism, then he is ethnicity is automatically Jew.