Source : Webster's Revised Unabridged Dictionary (1913)

Singleton \Sin"gle*ton\, n.
   In certain games at cards, as whist, a single card of any
   suit held at the deal by a player; as, to lead a singleton.

Source : WordNet®

singleton
     n 1: a single object (as distinguished from a pair)
     2: a set containing a single member
     3: the playing card that is the only card in a suit held in a
        bridge hand as initially dealt