An organized private event where guests can engage in BDSM or sex within agreed house rules and explicit consent.
The word doing the heavy lifting here is "play." In BDSM, play means any negotiated kink activity between consenting adults — it does not automatically mean sex. So a play party is the event built around that: a room (often a rented dungeon, club, or someone's carefully prepped flat) with a socialising area, a space to change into gear, and a designated play area with equipment. You can spend the whole night talking and watching and never touch anyone. That is normal, and it is the point.
The common misconception is that "play party" is a polite synonym for "orgy." It isn't. A sex party is organised around sex; a play party is organised around scenes — the negotiated, often non-penetrative exchanges of BDSM. Plenty of play happens fully clothed. Conflating the two is exactly the mistake that gets people to misread the etiquette, and the etiquette is the whole architecture.
That etiquette is stricter than most first-timers expect, and deliberately so. Consent is explicit and specific, never inferred from a look or the fact that someone showed up: you ask before you touch anyone or anything — a person, their rope, their toys. You never walk through or interrupt a scene in progress. Phones and cameras stay off in play space. What you see there stays there. Larger events run dungeon monitors — volunteers who enforce the rules — precisely because the freedom of the space depends on those lines holding.
One honest footnote on the name: "play party" is much older than kink. In 19th-century rural America it meant a gathering of singing-and-dancing games, a workaround for religious communities that banned dancing. The BDSM scene did not invent the phrase; it reused the same two words for its own kind of "play."