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.