Definition:Doubleton

Definition
A doubleton is a set that contains exactly two elements.

The doubleton containing the elements $a$ and $b$ can be written $\left\{{a, b}\right\}$.

The set $\left\{{a, b}\right\}$ is known as the doubleton of $a$ and $b$.

It is also known as an unordered pair.

Also see

 * Axiom of Pairing