In mathematics, a singleton is a set with exactly one element. For example, the set {0} is a singleton. Note that a set such as is also a singleton: the only element is a set (which itself is however not a singleton).
A set is a singleton if and only if its cardinality is 1. In the set-theoretic construction of the natural numbers, the number 1 is defined as the singleton {0}.
In axiomatic set theory, the existence of singletons is a consequence of the axiom of empty set and the axiom of pairing: the former yields the empty set {}, and the latter, applied to the pairing of {} and {}, yields the singleton .
If A is any set and S is any singleton, then there exists precisely one function from A to S, the function sending every element of A to the one element of S.
Structures built on singletons often serve as terminal objects or zero objects of various categories:
- The statement above shows that every singleton S is a terminal object in the category of sets and functions. No other sets are terminal in that category.
- Any singleton can be turned into a topological space in just one way (all subsets are open). These singleton topological spaces are terminal objects in the category of topological spaces and continuous functions. No other spaces are terminal in that category.
- Any singleton can be turned into a group in just one way (the unique element serving as identity element). These singleton groups are zero objects in the category of groups and group homomorphisms. No other groups are terminal in that category.
