An action that happens with the free consent of all persons affected is called consensual. The term is mostly used in situations that can appear to outsiders as if one person was forcing something on another person against that person's will, such as all kinds of BDSM play.
Consensuality is one of the key aspects that distinguishes BDSM activities from domestic violence, maltreatment, torture and abuse. This is why SSC (Safe, Sane and Consensual) is an important maxim of BDSM.
The opposite, non-consensual (often abbreviated NC), means that something (usually something unpleasant) is imposed on a person by force and against his or her will. There are cases where non-consensual actions are perfectly legal: for example, when a person is arrested by the police, or when a prisoner is subjected to judicial punishment.
Non-consensuality is a popular theme in BDSM and spanking literature. As there are people who don't like to read stories with non-consensual plot elements, it has become a common practice to place "NC" as a warning in the subject lines of such postings or in the headers of such stories.