Morse–Kelley set theory

Morse–Kelley set theory is a form of set theory that serves as an alternative foundation for mathematics. It is an extension of Zermelo-Fraenkel set theory (ZF) that includes classes, similar to von Neumann–Bernays–Gödel (NBG) set theory. The primary distinguishing feature of Morse–Kelley set theory is its treatment of proper classes, which are collections that are too large to be considered sets within the framework.