Regular cardinal
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An infinite cardinal number κ is called regular if cf(κ) = κ, where cf is the cofinality operation. This says that κ cannot be expressed as the union (supremum) of a collection of less than κ smaller cardinals. If we demand a regular cardinal be also a limit cardinal, we get an inaccessible cardinal.
Infinite cardinals which are not regular are called singular (the existence of singular cardinals requires the Axiom of replacement.)