An agreed upon word, or set of words, used in any kind of sexual activity, not limited to BDSM play, that is uttered by any participant to communicate that they want the activity to be modified.
Sexual safe words could also be called "sexual trust words", and they're an especially good idea in any kind of sex play that is exploratory and/or new to any participant.
Different safe words can be created and agreed upon to mean different things, such as, "stop everything, right now!", "slow down, back off!", "don't stop!", etc.
Safe words should be ones that a participant isn't likely to use as part of the play. They can be complete code, like "Tibet" (stop), "Chicago" (slow down), and "Glesga" (don't stop), or more obvious, like "red", "yellow", and "green".
No kind of sex should EVER be forced on anyone- that's called rape! (which is totally different than indulging in an agreed upon rape fetish)- and the basic idea of safe words is to prevent rape from happening accidentally.
Safe words make for hot sex!
Thank god we had a safe word, the idjit hadn't met a nail file in his whole fucking life!
"Yes, yes, more, more, Glesga, Glesga, please, give it to me, give it to me!" (Glesga is my favorite safe word, dontcha know, Sweetie?).
I could barely speak, but the safe word was easy to remember, so, when she went from four fingers to all five I managed to gasp, "yellow"- then a moment later I was screaming, "green! green! green!".
In BDSM community, a word (usually irrelevant and strange in the context of the sexual situation) agreed by the participating parties to cease the activity. This is so that the submissive partner(s) can say "stop" and "no" as often as they want during the session and use the safe word when they actually mean it.
Top: *whip whip*
Bottom: Ouch, no, stop, please stop ... FISHSTICKS!
Bottom: Dammit, that's our safe word - now seriously, fishsticks!
In the practice of sado-masochism, a "safe word", is an agreed upon word uttered by the recipient masochist, when he/she can no longer tollerate any more pain administered by the sadist, during a session. Maybe also be used, to indicate they may need a temporary "break" in said session.