■ noun
1》 a single person or thing of the kind under consideration.
↘a child or animal born singly, rather than one of a multiple birth.
↘(in card games, especially bridge) a card that is the only one of its suit in a hand.
2》 informal a person who is not in a long-term relationship.
3》 Mathematics & Logic a set which contains exactly one element.
Origin
C19: from single , on the pattern of simpleton .