Safe is being knowledgeable about the techniques and safety concerns involved in what you are doing, and acting in accordance with that knowledge.
While the media often portrays the more extreme SM behaviors, the reality is that a lot of SM play never goes beyond a playful spanking. Just as there are ways to reduce the risk in activities such as scuba diving or driving a car, there are ways to reduce the risk and engage in SM behavior safely.
Sane is knowing the difference between fantasy and reality. Fictional accounts of S+M are often distorted for fantasy sake, and are not representative of real situations and relationships.
Sane also distinguishes between mental illness and health. A real distinction between mental illness and health is when a behavior pattern causes problems in a persons personal life.
As is washing your hands until the skin is peeling off, or so frequently that you can not otherwise function is a sign mental illness. S+M, like any other behavior, can be a sign of psychiatric problems. However the VAST majority of its practitioners find that SM enriches and promotes functionality in the other areas of their life. (Unfortunately, there are a few exceptions, and know a few).
Consensual is respecting the limits imposed by each participant at all times.
Consent is the prime ingredient of S+M. One difference between rape and heterosexual intercourse is consent. One difference between violence and S+M is consent. The same behaviors that might be crimes without consent are life-enhancing with consent.
The type and parameters of control are agreed upon by the people involved, and the ongoing consent of all participants is required.
Some practitioners use a safeword, which is a designated word that signals the scene must slow down or stop.