- In Mathematics , a is a Set with Exactly One element. See Singleton (mathematics) .
- In Computer Programming , a is a common Design Pattern . It refers to a Class which is meant to be not freely instantiated, but have only a defined number of instances. See Singleton Pattern . A 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 is a character encoded with one unit, as opposed to characters encoded with multiple-unit sequences.
- In Unicode , a 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 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 . 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 describes the last cask or charge of a Whisky .
- In Espionage , a is an Intelligence Officer who works alone, in that he has no supporting network.
- In science-fiction stories involving multiple parallel realities, a 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 is a radar or visual contact confirmed to be a single aircraft.
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