Autoerotic
Autoeroticism is the practice of fulfilling one's own sexual needs without a partner. The most common form of autoeroticism is masturbation. Many people also use dildos, vibrators, anal balls, sybian machines, and other sex toys alone.
Most autoerotic practices are relatively or entirely safe.
One example of a dangerous practice is autoerotic asphyxiation. It often results in death, particularly when done alone.
Self-bondage is the practice of sexual bondage by oneself; it carries a higher risk than bondage with a partner, and there is a large potential for things to go wrong.
