Associated with copulation of more than one partner outside their shared monogamous bond, as in "wife-swapping" (why don't they ever call it "husband-swapping"?) or couples who have sex with members of other couples. (Not quite the same thing as polyamory, though -- at least from the point of view of most polyamorists, who say that polyamory is about loving more than one person and swinging is about having sex with more than one person, with love discouraged outside the monogamous pair.)