This sequential mixed-methods study examines how Americans ascribe meanings to the concepts racism, sexism, and classism. We first conduct interviews (N = 40) using a symbolic boundaries elicitation approach, gathering examples of scenarios that do and do not "count" as racism, sexism, and classism. We then use these examples as vignettes in a nationally representative survey experiment (N = 2,000). Results reveal striking evidence for cultural heterogeneity in how Americans understand and define racism, sexism, and classism. We find that a person's definition of these concepts depends on their emphasis on intentionality, unequal treatment/outcomes, and power (a)symmetry. We also find that political partisanship, gender, age, and income shape the importance of these three components in their definitions. Finally, we show that Americans' definitions of racism, sexism, and classism strongly predict their discrimination-related public opinion and policy preferences, such as support for affirmative action and antidiscrimination laws, even after accounting for demographic controls, including political views.