Sociologist William Graham Sumner popularized the term in his 1906 book "Folkways," describing the view that one's own group is the center of everything and the standard for judging others. Every group can be ethnocentric, but in the US it carries the most weight when the dominant culture's norms are built into institutions.
Ethnocentrism shapes what counts as normal in schools, workplaces and law. It appears in curricula that treat European history as world history, in professional dress codes that penalize Black hairstyles, and in judgments that call other languages or accents uneducated. US immigration law put it into statute: the Immigration Act of 1924 set national origin quotas designed to favor Northern and Western Europeans, and they lasted until 1965.
Ethnocentrism differs from racism in that it centers culture rather than a belief in biological hierarchy, but the two often work together. Remedies include ethnic studies, cultural humility and policies that stop treating one group's norms as neutral.