Kinktionary
More like guidelines and a lot less like definitions.
Consensual Non-Consent
A form of consent where the people participating agree and consent to that certain words are to be ignored, or that there is no form of consent needed for activities during a scene. Often abbreviated to CNC.
In some forms of CNC, safety measures like safe words are not in place. While limits may still be set beforehand, vocal cues, like a "no", "stop", or even "red", may be ignored, usually because the other person wants to use them to role play and desires them to be ignored. CNC is often seen as playing with a safe situation that a person may not fully desire or want, but still wants to have the experience of, and has consented to exploring it with the person / people involved.
People may use CNC to re-live trauma experiences in an environment where they have made the choice to experience it or that still allows them a way to get out of the situation in a safe manner. This triggering can result in negative scenarios, but this will have to be discussed and prepared beforehand with the people involved.
CNC in a crowd
Note that CNC play in public places can be problematic, as bystanders may not be aware of the dynamic and the CNC that has been agreed upon. As it can look a lot like a consent violation, it can also trigger people that have experiences with consent violations or similar 'real' scenarios. Because of that many people that have these scenes in public will do this in a controlled manner where everyone is aware of the situation and has been informed about the agreement before the scene begins.