What defines a ethnicity is shared cultural traditions, religious beliefs, the type of food that is eaten, language, common ancestry, type of music, region of origin, history, & experiences.
new ethnicities have been created over time such as the Cajuns, Islenos, Neomexicanos, Mexican Mestizos, Scots-Irish or Ulster Scots, Irish Catholics, Heinz 57, Euro-American Mutts, Tejanos, Caliornios, Chicanos or Mexican-Americans, African-Americans, Gullahs, Louisiana Creoles, Afro-Caribbeans, Afro-Latinos, WASPs or Old-Stock Americans, Mexican Mestizos, French-Canadians, Guachos, Pardos in Brazil, Metis, Afrikaners, Coloureds in South Africa, Anglo-Celts in Australia, & many others.
Ethnicity is both your ancestry & how you self-identify, it's a social construction, it has no genetic basis unlike race.