Although the term “ethnic group” (EG) is often used in social studies, its definition differs among researchers. Moreover, little is known about ordinary people’s subjective understanding of this term, even though it is often used in social discourse. We examined this issue in a cross-sectional study of 273 American, British, Mexican, and Polish students using an open-ended questions approach. Results revealed cultural differences in patterns of “ethnic group” definitions across the four countries. U.S. respondents predominantly connected EG to “race”; British participants frequently related it to “race,” but more often to “common culture” and “customs/traditions.” Both latter categories were overwhelmingly dominant in Mexico and Poland. However, “nation,” “shared history,” “religion,” “language,” and “territory” were also very popular as EG understandings in Poland. Although most participants used the newer definition of EG (referring to all groups in a society, including minority and majority groups), a few in each country used the term only to refer to minorities and people different from themselves (an older, “minus one” definition). Unexpected definitions of EG also appeared (e.g., people having similar hobbies, having similar work goals, or living in the same city). The results also indicate that for the United States, the United Kingdom, and Mexico, “ethnic group” was more a subgroup within a nation, whereas in Poland, they represented the same level of categorization. The theoretical and practical implications of our findings are discussed.