1. Theoretical particle in physics usually connected with cosmic rays. Its name comes from an ancient Greek word for "speed", because it was thought to travel very fast. The particle has never been observed, and the name gradually fell out of favor in conventional physics. More recently, some researchers, inventors, and marketers have revived the term without defining it, which has led to some confusion.

2. Mutated particle, harmful to humans, which is created when a type of cosmic ray strikes the Earth and comes out the other side. Anyone who spends much time at a spot where these particles come out will have serious health problems. (Vince Wiberg)