Oxford English Dictionary
Etymology: < Spanish casta and Portuguese casta ‘race, lineage, breed’ (Minsheu); originally according to Diez ‘pure or unmixed (stock or breed)’, < casta feminine of casto < Latin castus -a pure, unpolluted (cf. chaste). . . .
1.a. A people, stock, or breed (of men). . . .
b. [ < Spanish casta .] In South America: any of various groups of people of mixed descent.
2.a. spec. One of the several hereditary classes into which society in India has from time immemorial been divided; the members of each caste being socially equal, having the same religious rites, and generally following the same occupation or profession; those of one caste have no social intercourse with those of another. (The original casts were four: 1st, Brahmans or priestly caste; 2nd, the Kshatriyas or military caste; 3rd, the Vaisyas or merchants; 4th, the Sudras, or artisans and labourers. These have in the course of ages been sub-divided into an immense multitude, almost every occupation or variety of occupation having now its special caste. This is now the leading sense, which influences all others.
b. transf. A hereditary class resembling those of India. fig. A class who keep themselves socially distinct, or inherit exclusive privileges.
3.b. gen. and fig. A system of rigid social distinctions in a community; to lose caste : to lose social rank, to descend in the social scale
Gunnar Myrdal, in the influential mid-20c study, An American Dilemma: The Negro Problem and Modern Democracy (1944) described white and black people in the United States as two distinct "castes" who could not interact with each other as equals, thus prohibiting intermarriage and other processes of assimilation.