Generally, a singleton is something which exists alone in some way. The word is used in several different areas:
Singleton is used to refer to single babies (as opposed to twin or other multiple births)
In mathematics, a singleton is a set with exactly one element, for example: {1}. Note that a set such as {{1,2,3}} is also a singleton: the only element is a set (which itself is not a singleton). In axiomatic set theory, the existence of singletons is a consequence of the axiom of pairing.
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 the game of contract bridge, a singleton has a specific meaning referring to a hand containing only one card of a particular suit, 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.
... families residing in the town. The population density is 1,322.5/km² (3,428.5/mi²). There are 4,661 housing units at an average density of 437.9/km² ...