Definition:Regular Cardinal

Definition
Let $\kappa$ be an infinite cardinal.

Then, $\kappa$ is a regular cardinal iff $\operatorname{cf} \left({ \kappa }\right) = \kappa$, that is, the cofinality of $\kappa$ is equal to itself.

Also see

 * Singular Cardinal
 * Weakly Inaccessible Cardinal
 * Strongly Inaccessible Cardinal