Generally, a singleton is something which exists alone in some way. The word is used in several different areas:
- Singletons are single babies (as opposed to twins or other multiple births)
- In mathematics, a singleton is a set with exactly one element. See singleton (mathematics).
- In computer programming, a singleton is a common design pattern. It refers to a class which is meant to be instanced once, and only once. It is much like a global variable in functional programming, but a "global class-instance" instead. See Singleton pattern.
- In the wake of the popularity of the novel and movie Bridget Jones's Diary, the word has also become popular as a (self)description of individuals without romantic partners, particularly applied to 20/30-something females. There is an undertone to this use of ignoring societal pressures towards partnerships.
- In trick-taking games where suit is important, such as contract bridge, Hearts, and Ambition, if a hand contains only one card of a particular suit, that card is called a singleton. Certain bidding systems (notably Acol) add notional value to the hand for a short suit of this type since it exposes the possibility of trumping. Similarly, holding a "singleton Queen" (of Spades) in Hearts is a very disadvantageous position.
- Singleton, Australia, a place in New South Wales