A person who has un-planned sexual encounters with numerous people who they are not dating and do not intend to date.
A person can be a swinger while single and faithful while in a relationship.
On the other hand, there are some people who continue to swing even while in relationships.
Person1- 'The other night I hooked up with 3 different men/women.'
Person2- 'That's disgusting, you filthy swinger.'
Person3- 'Sounds like good times! I wish I were a swinger!'