Racism is the belief that groups of humans possess different behavioral traits corresponding to inherited attributes and can be divided based on the superiority of one race over another. It may also mean prejudice, discrimination, or antagonism directed against other people because they are of a different race or ethnicity. Modern variants of racism are often based in social perceptions of biological differences between peoples. These views can take the form of social actions, practices or beliefs, or political systems in which different races are ranked as inherently superior or inferior to each other, based on presumed shared inheritable traits, abilities, or qualities. There have been attempts to legitimize racist beliefs through scientific means, such as scientific racism, which have been overwhelmingly shown to be unfounded. In terms of political systems (e.g. apartheid) that support the expression of prejudice or aversion in discriminatory practices or laws, racist ideology may include associated social aspects such as nativism, xenophobia, otherness, segregation, hierarchical ranking, and supremacism.

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). Racism and racial discrimination are often used to describe discrimination on an ethnic or cultural basis, independent of whether these differences are described as racial. According to the United Nations's Convention on the Elimination of All Forms of Racial Discrimination, there is no distinction between the terms "racial" and "ethnic" discrimination. It further concludes that superiority based on racial differentiation is scientifically false, morally condemnable, socially unjust, and dangerous. The convention also declared that there is no justification for racial discrimination, anywhere, in theory or in practice.

Racism is a relatively modern concept, arising in the European age of imperialism, the subsequent growth of capitalism, and especially the Atlantic slave trade, of which it was a major driving force. It was also a major force behind racial segregation in the United States in the 19th and early 20th centuries, and of apartheid in South Africa; 19th and 20th-century racism in Western culture is particularly well documented and constitutes a reference point in studies and discourses about racism. Racism has played a role in genocides such as the Holocaust, the Armenian genocide, the Rwandan genocide, and the Genocide of Serbs in the Independent State of Croatia, as well as colonial projects including the European colonization of the Americas, Africa, Asia, and the population transfer in the Soviet Union including deportations of indigenous minorities. Indigenous peoples have been—and are—often subject to racist attitudes.