Non-consensual, also spelled nonconsensual and abbreviated NC or N/C, means that at least one of the participants in a certain activity is being forced to engage in that activity against his or her will. For example, rape is a non-consensual form of sexual intercourse, because the victim is being forced to have sex.
When used in reference to spanking, non-consensuality means that the spankee is forced to accept the punishment. This is the normal situation in any spanking given for real punishment (disciplinary spanking) and not for the fun or erotic pleasure of the participants. Nearly all spanking of children, whether domestic spanking or school spanking, is non-consensual, and so is judicial corporal punishment. The very nature of punishment is that it is imposed on the delinquent against his or her will.
A consensual spanking, on the other hand, is one where the spankee submits freely to the spanking. This is the normal case in BDSM and spankophile forms of spanking such as fun spanking, sensual spanking, erotic spanking or therapeutic spanking.
In the case that the spankee is submissive and accepts their punishment cooperatively and obediently, it can be a little difficult to decide whether the spanking is consensual or non-consensual because there is little or no force visible. But in the general, a spanking is only called consensual when the spankee consents completely freely without any pressure or fear; in all other cases it is called non-consensual even if the spankee co-operates without any protest.
While the spankophile community shares the ideal of full consensuality in all real life scenarios (SSC), non-consensuality plays a central role in spanking fiction. "NC" is used in the subject lines of BDSM and spanking stories to inform and warn the reader that the story features non-consensual BDSM or spanking action.