Recreational, sexual non-monogamy — a distinct subculture with its own lineage, separate from polyamory.
Swinging is the branch of non-monogamy that keeps sex and love in separate rooms. The classic unit is a couple: the bond between the two is the anchor, and the play with others is recreation, not romance. That's the line people draw against polyamory — poly builds several loving relationships; swinging mostly builds a very good Saturday. Plenty of folks live somewhere in between, which is exactly why the scene treats arguing over definitions as a hobby.
It runs on its own machinery: clubs, house parties, cruises, conventions, and a whole grammar of screening, boundary-setting and invitation. There's even a discreet signal — an upside-down pineapple — that quietly says "we play" from a hotel bar. Ground rules get negotiated up front, not improvised at 2am, and "no" is a full sentence.
The lazy stereotype is seedy carpet and 1970s desperation. The research says otherwise: sociologists who've actually studied the lifestyle tend to describe people who are educated, articulate about what they want, and unusually deliberate about consent. Gayle Rubin's "charmed circle" is useful here — swingers break one big social rule (monogamy) while carefully keeping another (a committed primary bond), which is why they manage to unsettle the vanilla world and the poly world at the same time. Radical and conservative in one breath.