Racism is the belief that there are inherent differences in people's traits and capacities that are entirely due to their race, however defined, and that, as a consequence, racial discrimination (i.e. different treatment of those people, both socially and legally) is justified.
In the modern English language, the term "racism" is used predominantly as a pejorative epithet. It is applied especially to the practice or advocacy of racial discrimination of a pernicious nature (i.e. which harms particular groups of people), and which is often justified by recourse to racial stereotyping or pseudo-science. Modern usage, in fact, often equates "racism" and "racial discrimination" and defines the latter term only as applying to pernicious practices. Differential treatment of racial groups that is intended to ameliorate past discrimination, rather than to harm, goes by other names (e.g. affirmative action); the characterization of this practice as "racism", "racial discrimination" or "reverse discrimination" is normally only done by its opponents, and typically implies a belief in the harmful nature of the practice with respect to the groups not receiving assistance.
Racism is popularly associated with various activities that are illegal or commonly considered harmful, such as extremism, hatred, xenophobia, (malignant or forced) exploitation, separatism, racial supremacy, mass murder (for the purpose of genocide), genocide denial, vigilantism (hate crimes, terrorism), etc.
"Racism" and "racial discrimination" are often used to describe discrimination on an ethnic or cultural basis, independent of their somatic (i.e. "racial") differences. According to the United Nations conventions, there is no distinction between the term racial discrimination and ethnicity discrimination.