a single object (as distinguished from a pair)
"After sorting through the drawer, I found that my favorite pen was the only remaining singleton among a stack of broken ones."
a set containing a single member
"The mathematician proved that every singleton subset of an infinite set shares the same cardinality as any other singleton subset."
the playing card that is the only card in a suit held in a bridge hand as initially dealt
"After being dealt his cards, Mark immediately discarded the singleton spade to avoid wasting a trick on an unsupported lead."
A playing card that is the only one of its suit in a hand, especially at bridge.
"After discarding his last hearts, I realized my singleton was now useless since no partner could ruff it."
A surname.
"The local news report featured a story about the Singleton family's new community garden project."