Racism is, at minimum, the construction and ranking of human groups along supposedly biological or essential lines, and the use of that ranking to justify unequal treatment. The fuller analytical use of the word — now standard in serious social and historical scholarship — distinguishes several layers: racial ideology (the set of beliefs that naturalize the ranking), individual racism (interpersonal prejudice and discrimination), institutional or structural racism (the disparate effects produced by ostensibly neutral institutional rules — in housing, lending, policing, schooling, sentencing, healthcare), and systemic racism (the reproduction of these effects across institutions over time, even as overt ideology weakens or changes).
Historically, modern racism emerged in tandem with the European expansion of the 15th–17th centuries and crystallized around the Atlantic slave trade and the conquest of the Americas — a sequence that, Cedric Robinson and others have argued, makes race and capitalism inseparable in their formation. The pseudo-scientific racisms of the 19th and early 20th centuries (phrenology, eugenics, Nazi racial law) gave the ideology a particular form whose collapse after 1945 did not end racism but reshaped its modes — toward what scholars variously call colorblind racism, caste, or white supremacy understood as a structure rather than an avowed ideology.
In the United States, the long Black intellectual tradition — W.E.B. Du Bois, James Baldwin, Ralph Ellison, Toni Morrison, Isabel Wilkerson, Jelani Cobb — has insisted that racism is not a deviation from the American project but constitutive of it, and that any honest political accounting of the country has to begin there.