The term describes a shift in the origin of those entering the United States, primarily occurring from the 1880s to 1920. Individuals arrived increasingly from Southern and Eastern European countries, rather than primarily from Northern and Western Europe as in earlier periods. These areas included Italy, Russia, Poland, and Greece. A defining characteristic was often cultural and religious differences compared to the established American population; many practiced Catholicism, Orthodox Christianity, or Judaism, and spoke languages other than English.
This demographic shift is significant because it fueled nativist sentiments and contributed to social tensions. Increased immigration led to debates about assimilation, cultural identity, and the economic impact of a large, often unskilled, labor force. Legislative actions, such as immigration quotas, were enacted to restrict further immigration from these regions. This period fundamentally reshaped the nations ethnic and religious composition, leaving a lasting impact on American society and politics.