Racism is discrimination and prejudice against people based on their race or ethnicity. Racism can be present in social actions, practices, or political systems (e.g. apartheid) that support the expression of prejudice or aversion in discriminatory practices. The ideology underlying racist practices often assumes that humans can be subdivided into distinct groups that are different in their social behavior and innate capacities and that can be ranked as inferior or superior. Racist ideology can become manifest in many aspects of social life. Associated social actions may include nativism, xenophobia, otherness, segregation, hierarchical ranking, supremacism, and related social phenomena. Racism refers to violation of racial equality based on equal opportunities (formal equality) or based on equality of outcomes for different races or ethnicities, also called substantive equality.

While the concepts of race and ethnicity are considered to be separate in contemporary social science, the two terms have a long history of equivalence in popular usage and older social science literature. "Ethnicity" is often used in a sense close to one traditionally attributed to "race", the division of human groups based on qualities assumed to be essential or innate to the group (e.g. shared ancestry or shared behavior).