In set theory, a regular cardinal is a cardinal number that is equal to its own cofinality. More explicitly, this means that ${\displaystyle \kappa }$ is a regular cardinal if and only if every unbounded subset ${\displaystyle C\subseteq \kappa }$ has cardinality ${\displaystyle \kappa }$. Infinite well-ordered cardinals that are not regular are called singular cardinals. Finite cardinal numbers are typically not called regular or singular.

In the presence of the axiom of choice, any cardinal number can be well-ordered, and so the following are equivalent:

Crudely speaking, this means that a regular cardinal is one that cannot be broken down into a small number of smaller parts.

The situation is slightly more complicated in contexts where the axiom of choice might fail, as in that case not all cardinals are necessarily the cardinalities of well-ordered sets. In that case, the above equivalence holds for well-orderable cardinals only.

An infinite ordinal ${\displaystyle \alpha }$ is a regular ordinal if it is a limit ordinal that is not the limit of a set of smaller ordinals that as a set has order type less than ${\displaystyle \alpha }$. A regular ordinal is always an initial ordinal, though some initial ordinals are not regular, e.g., ${\displaystyle \omega _{\omega }}$ (see the example below).

The ordinals less than ${\displaystyle \omega }$ are finite. A finite sequence of finite ordinals always has a finite maximum, so ${\displaystyle \omega }$ cannot be the limit of any sequence of type less than ${\displaystyle \omega }$ whose elements are ordinals less than ${\displaystyle \omega }$, and is therefore a regular ordinal. ${\displaystyle \aleph _{0}}$ (aleph-null) is a regular cardinal because its initial ordinal, ${\displaystyle \omega }$, is regular. It can also be seen directly to be regular, as the cardinal sum of a finite number of finite cardinal numbers is itself finite.

${\displaystyle \omega +1}$ is the next ordinal number greater than ${\displaystyle \omega }$. It is singular, since it is not a limit ordinal. ${\displaystyle \omega +\omega }$ is the next limit ordinal after ${\displaystyle \omega }$. It can be written as the limit of the sequence ${\displaystyle \omega }$, ${\displaystyle \omega +1}$, ${\displaystyle \omega +2}$, ${\displaystyle \omega +3}$, and so on. This sequence has order type ${\displaystyle \omega }$, so ${\displaystyle \omega +\omega }$ is the limit of a sequence of type less than ${\displaystyle \omega +\omega }$ whose elements are ordinals less than ${\displaystyle \omega +\omega }$; therefore it is singular.

${\displaystyle \aleph _{1}}$ is the next cardinal number greater than ${\displaystyle \aleph _{0}}$, so the cardinals less than ${\displaystyle \aleph _{1}}$ are countable (finite or denumerable). Assuming the axiom of choice, the union of a countable set of countable sets is itself countable. So ${\displaystyle \aleph _{1}}$ cannot be written as the sum of a countable set of countable cardinal numbers, and is regular.