Safe, sane and consensual (SSC) is one of several phrases used by a large section of the BDSM and sexual bondage communities to describe themselves and their philosophies who regard SSC to be a watchword for safety.
The principles are that BDSM activities should be:
safe: attempts should be made to identify and prevent risks to health sane: activities should be undertaken in a sane and sensible cast of mind consensual: all activities should involve the full consent of all parties involved, but note that legal consent may not create a defence to criminal liability for any injuries caused and that, for these purposes, non-physical injuries are included in the definition of grievous bodily harm in English law. Other people in the BDSM community do not consider SSC to be an accurate term for these relationships/activities. The term Risk Aware Consensual Kink (RACK) is sometimes used as a substitute description.
For those who dissent, issues generally arise from the subjective nature of each term in SSC when using the term as a yardstick to evaluate activities. It can be argued that the terms in RACK are equally subjective. "How safe is safe enough" is a legitimate question, and "how aware is aware enough" is an equally legitimate question.
Most attribute the term to David Stein who coined it in 1984 for GMSMA. More information can be found in the essay titled Safe Sane Consensual: The Evolution of a Shibboleth, in which he states that the term was developed "to distinguish the kind of S/M I wanted to do from the criminally abusive or neurotically self-destructive behavior popularly associated with the term 'sadomasochism'."