A word that a sub in a session can use to stop the session. Quite frequently in play, particularly in BDSM play, partners which to be able to have a scene which is consensual non-consensual. In other words, they want to be able to say "no" and not have it mean "stop." But there has to be a way to communicate when things get too much, so there are Safewords. Partners agree before the session that when the Sub says certain words, the Top will respect those words and stop.
Quite commonly, RED, YELLOW, and GREEN are used as safewords.
Their meanings (typically):
Green: The only way a sub may convey that what is happening is very pleasurable or an "I'm okay."
Yellow: "This is okay, but I am at the edge of what I can take."
Red: STOP. Now.