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, iff the cofinality of $\kappa$ is equal to itself.

Also see

 * Singular Cardinal
 * Weakly Inaccessible Cardinal
 * Strongly Inaccessible Cardinal