Swinging

Swinging is a relationship where a couple has recreational sex with other people.

There many synonyms used with swinging, including "swapping" and "the lifestyle".

They key difference between swinging and polyamory is that swingers intentionally set boundaries around the level of intimacy involved with other swingers. This limits some of the complexity that an arise with polyamorous relationships.

These are some broad categories that are helpful to understand.

For a more complete picture of all the ways you can define your relationships, check out this graphic.