Safeword

In BDSM, a safeword is a code word, series of code words or other signal used by a submissive or bottom to unambiguously communicate their physical or emotional state to a dominant or top, typically when approaching, or crossing, a physical, emotional, or moral boundary. Some safewords are used to stop the scene outright, while others can communicate a willingness to continue, but at a reduced level of intensity. Safewords are usually agreed upon before playing a scene by all participants, and many organized BDSM groups have standard safewords that all members agree to use to avoid confusion at organized play events.

Safewords of BDSM fall under the guiding philosophy of safe, sane and consensual. Those who practise the more permissive philosophy of risk-aware consensual kink may abandon the use of safewords, especially those that practice forms of edgeplay or extreme forms of dominance and submission. In such cases, the choice to give up the use of safewords is a consensual act on the part of the bottom or submissive.

Safewords are usually used by the submissive, but can be used by all participants in a scene, including tops, dungeon masters at play parties, and sometimes even observers. For example, a submissive may misbehave intentionally to indicate the desire for harsher treatment, and sometimes a top will need to safeword the scene to let them know it has gone too far for the top to continue the scene. Or, a third party observing a scene may have the ability to spot something dangerous going on that both the top and submissive have missed, and need to stop the scene to point it out.

A safeword makes it possible for a submissive to say "No" or "Stop" and pretend as much as he or she wants without really meaning it while still having a safe way of indicating they seriously need the scene to stop. In theory a safeword is usually a word that the person would not ordinarily say during sex, such as pineapple, velociraptor, or teacup. With the range of safewords in common use it is important that the safeword be negotiated beforehand.

Since a scene may become too intense for a submissive partner to remember what the safeword is, in practice commonly the words safeword or red are also used as safewords. They are often the default at many play parties, or respected as a safeword in addition to any negotiated safeword. A dungeon monitor would likely expect either of those words to be respected.

...
Wikipedia