WordNet (r) 3.0 (2006):

ethnic group
    n 1: people of the same race or nationality who share a
         distinctive culture [syn: ethnic group, ethnos]

Moby Thesaurus II by Grady Ward, 1.0:

29 Moby Thesaurus words for "ethnic group":
   breakaway group, camp, caucus, community, culture, division,
   faction, interest, interest group, minority group, nation,
   nationality, offshoot, party, people, political party,
   pressure group, race, sect, side, silent majority, society,
   speech community, splinter, splinter group, stock, strain,
   vocal minority, wing