1. the notion that conflict between social and economic classes is a fundamental determining force in human affairs, affecting not only systems of government and social organization but also individual psychology. It is held that one’s perceptions, goals, and expectations, and even one’s conceptions of psychological health and illness, are heavily influenced by the class of which one is a member. Most modern manifestations of class theory trace their origins to Karl Marx, although the work of the critical theorists of the Frankfurt school (1930s onward) has also been influential. See Marxism.

2. the branch of set theory that is concerned with the properties of classes. It is particularly interested in defining the distinction between classes and sets that are not classes.