noun

• (playing cards) A playing card that is the only one of its suit in a hand, especially at bridge.
• (playing cards) A hand containing only one card of a certain suit.
• A single object, especially one of a group.
• (computing) A class that may not be instantiated more than once, i.e. that implements the singleton design pattern.
• (mathematics) A set with exactly one element.
• A child or animal that is born singly, not as a twin or other multiple birth.
• A person without a romantic partner.
• A person without a dissociative identity.
• (phonetics) A single consonant, as opposed to a geminated consonant.
• (uncountable) A placename:
• A village and civil parish in Fylde borough, Lancashire, England (OS grid ref SD3838).
• A suburb in Great Chart with Singleton parish, Ashford borough, Kent, England (OS grid ref TQ9841).
• A village and civil parish in Chichester district, West Sussex, England (OS grid ref SU8713).
• A town north-west of Newcastle, New South Wales, Australia.
• An outer southern suburb of Perth, Western Australia.
• A habitational surname from Old English.