A caste is a fixed social group into which an individual is born within a particular system of social stratification: a caste system. Within such a system, individuals are expected to: marry exclusively within the same caste (endogamy), follow lifestyles often linked to a particular occupation, hold a ritual status observed within a hierarchy, and interact with others based on cultural notions of exclusion, with certain castes considered as either more pure or more polluted than others. Its paradigmatic ethnographic example is the division of India's Hindu society into rigid social groups. Its roots lie in South Asia's ancient history and it still exists. However, the economic significance of the caste system in India has been declining as a result of urbanisation and affirmative action programs. A subject of much scholarship by sociologists and anthropologists, the Hindu caste system is sometimes used as an analogical basis for the study of caste-like social divisions existing outside Hinduism and India. In colonial Spanish America, mixed-race castas were a category within the Hispanic sector, but the social order was otherwise fluid.

Social stratification refers to a society's categorization of its people into groups based on socioeconomic factors like wealth, income, race, education, ethnicity, gender, occupation, social status, or derived power. It is a hierarchy within groups that ascribe them to different levels of privileges. As such, stratification is the relative social position of persons within a social group, category, geographic region, or social unit.