Generally, a singleton is something which exists alone in some way. The word is used in several different areas:

• 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 instantiated once, and only once. It is much like a global variable in functional programming, but a "global class-instance" instead. See Singleton pattern. A singleton is also a variable that is only used once in a program, usually indicating a programming mistake. See singleton variable.
• In variable-width encoding schemes for computer character sets, a singleton is a character encoded with one unit, as opposed to characters encoded with multiple-unit sequences.
• In Unicode, a singleton is a character the decomposition of which consists of a single character (e.g. the "ohm sign" (Ω) is decomposed as the "Greek capital letter omega" (Ω)).
• A singleton is a baby that is not a twin or other multiple birth.
• 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 and Hearts, 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.
• In Whisky singleton describes the last cask or charge of a Whisky.
• "Singleton" is the title of a short story by Greg Egan

Places called Singleton

In Australia:

• Singleton, New South Wales
• Singleton , a suburb of Perth, Western Australia

In the United Kingdom:

• Singleton , Lancashire
• Singleton , West Sussex