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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 not meant to be freely instantiated, but have only one instance. 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 women in their thirties. There is an undertone to this use of ignoring societal pressures towards marriage and motherhood.
• 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.
• In espionage, a singleton is an intelligence officer who works alone, in that he has no supporting network.
• In science fiction stories involving multiple parallel universes, a singleton is a person, place, or thing that exists in only one reality, regardless of how alike said realities might be otherwise. This is often used as a rationale for travel between those worlds.
• Singleton is the title of a short story by Greg Egan.
• In conformal field theory, see singleton field.
• In air combat manoeuvering, a singleton is a radar or visual contact confirmed to be a single aircraft.
• In coding theory, see Singleton bound.
• In XML (and its subsets such as XHTML) coding, refers to an empty tag, or self-closing tag, such as
  or .

In Australia:

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

In the United Kingdom:

• Singleton, Lancashire
• Singleton, West Sussex
• Singleton, Kent a suburb of Ashford, Kent

In the United States

• Singleton's in Ludlow, Vermont, a general store

• Al Singleton
• Alex Singleton
• Alvin Singleton
• Benjamin "Pap" Singleton, (1809-1892), African American separatist leader
• Billy Singleton
• Charles Laverne Singleton
• Chris Singleton
• Christopher Singleton
• Dave Singleton
• David Singleton, Irish musician
• Harry P. Singleton, American author
• Henry Singleton
• James Singleton (basketball)
• John Singleton, American film director
• John Singleton, Australian entrepreneur
• Kenneth Jarrett Singleton An American author and playwright best known for his play called "Angelica and Francesca" released in (2006)
• Ken Singleton, former Major League Baseball player and sports commentator
• Kenneth Singleton
• Lawrence Singleton
• Matthew Singleton
• Michael Singleton
• Penny Singleton
• Roger Singleton-Turner
• Sandy Singleton
• Sasha Singleton
• Valerie Singleton, British TV presenter
• Walter K. Singleton (1944-1967), Medal of Honor recipient
• William Dean Singleton
• Zutty Singleton

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