- an electron or positron emitted from an atomic nucleus in a certain type of radioactive decay.
- a high-speed electron or positron emitted by a nucleus during radioactive decay or nuclear fission
- A high-speed electron or positron, especially one emitted from the nucleus of an atom in radioactive decay.
- A high-speed electron or positron, usually emitted by an atomic nucleus undergoing radioactive decay. Beta particles are given off naturally by decaying neutrons in radioactive atoms and can be created in particle accelerators. Beta particles have greater speed and penetrating power than alpha particles but can be stopped by a sheet of aluminum that is 2 to 3 mm thick. See more at radioactive decay.