A popular song released and sold (on any format) nominally on its own though usually having at least one extra track.
“The Offspring released four singles from their most recent album.”
One who is not married or does not have a romantic partner.
“He went to the party, hoping to meet some friendly singles there.”
adjective
Not accompanied by anything else; one in number.
“Can you give me a single reason not to leave right now?”
Not married or (in modern times) not involved in a romantic relationship without being married or not dating anyone exclusively.
“Forms often ask if a person is single, married, divorced or widowed. In this context, a person who is dating someone but who has never married puts "single".”