A pre-agreed word or signal that any participant can use to instantly pause or stop a scene.
The mechanics are simple, which is the point. The most common system is the traffic light: "red" stops everything now, "yellow" means slow down or check in, "green" means keep going. When someone's mouth is busy or gagged, the safeword becomes a safe-signal — usually an object held in the hand that gets dropped. You pick words that would never show up organically in a scene, which is exactly why "no", "stop" and "please" make terrible safewords: in a lot of play, those are part of the script.
Here's the misconception worth killing: a safeword does not make a scene safe. It's a floor, not a ceiling — a hard emergency brake for the moment negotiation didn't foresee. The real safety work happens beforehand, in the boring conversation about limits, health and what "yellow" should actually trigger. And a safeword is only ever as good as the partner honouring it: it puts the power to stop in the bottom's hands, but it depends entirely on a top who listens the instant it's said. A safeword ignored isn't a safeword — it's evidence.
On where it comes from: nobody can hand you a name. The word is a plain compound that surfaced in print around the late 1980s, but the practice took shape in the 1970s gay leather and S/M scene and became standard through the 1980s — the same scene that built the ethical scaffolding (negotiation, limits, the "Safe, Sane, Consensual" ethos formalised by New York's GMSMA in 1983) that safewords slot into. Crediting one inventor would be tidy and false. It's a community tool, worn smooth by use.