You've probably used it, or had it sprinkled on you at some time in your life. It's processed from a soft mineral compound of magnesium silicate, and is called talcum powder or just talc.
Talcum dusting powder is commonly used to reduce rashes and diaper irritation in babies and infants. But this practice is dangerous. It can result in the inhalation of significant amounts of powder, causing acute or chronic lung irritation, known as talcosis. However, this risk is readily avoidable as cornstarch powder is a safe and reliable alternative.