'). The precise definition of "class" depends on foundational context. In work on Zermelo-Fraenkel set theory, the notion of class is informal, whereas other set theories, such as von Neumann-Bernays-Gödel set theory, axiomatize the notion of "proper class", e.g., as entities that are not members of another entity.

A class that is not a set (informally in Zermelo-Fraenkel) is called a proper class, and a class