Do you mean a safe word?
Of course but you could also use a code word to kick things off.
Ohh, I understand now. That's true
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Not quite. It's when you consent to anything your partner wants, even if you don't want it (though generally people who do CNC like feeling like they're being forced and end up liking it anyways). Generdlly speaking, the consent is intended to last for a very long period of time or even indefinitely, rather than for a single instance.