 Cofinality

The cofinality of an ordinal $\alpha$ is the least order type of an unbounded subset of $\alpha$, equivalently the least ordinal $\beta$ admitting a cofinal function $\beta \to \alpha$.

Cofinality is a concept in set theory, specifically in the context of cardinals and their relationships. It refers to a property of unbounded sets, particularly in the context of infinite cardinals.