Word or series of words or signal used to advise the dominant of the needs of the bottom, as in stop.
A quick means of a bottom being able to transmit their desires to the top, such as to signify they are okay, that it is safe to continue full bore, or to slow down, but still okay to proceed. It can also be to stop immediately.
Noteworthy: Words generally used are color codes. Green for go ahead, do more, yellow to slow down but go ahead, and red to stop. These terms are agreed upon prior to engaging in the session.
A top can also use these words to ascertain the condition of the bottom, by asking what their color code is.
In instances where a verbal confirmation is not possible, other signals or signs can be used, such as a specific pattern of blinking, of hand movements, or even simply the top placing their finger in the palm of the bottom, who either squeezes it or not, indicating a particular action.
Third parties witnessing can also use the Safeword, to stop a particular session if they spot something the participants don't notice.
Dungeon Masters also can have a Safeword, as can a top.
The use for a top is for when a bottom may be playing the part to misbehave, to guarantee harsher treatment. The top uses the Safeword to signal that they are not comfortable in going that far, in the administration of control.
Discussion prior to engaging in any 'extreme sex' is important, to not just determine the safety signals, words, but to also establish that there is no blame attached for their use. This is vital, to insure one or the other doesn't go beyond their limits, simply to please the other person.