Definition:Regular Cardinal

Definition
Let $\kappa$ be an infinite cardinal.

Then $\kappa$ is a regular cardinal :
 * $\operatorname{cf} \left({\kappa}\right) = \kappa$

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

Also see

 * Definition:Singular Cardinal
 * Definition:Weakly Inaccessible Cardinal
 * Definition:Strongly Inaccessible Cardinal