Beta particle, one of the particles that can be emitted by a radioactive atomic nucleus. Most are high-speed electrons, with a mass off about 1/1837 and a negative charge. More rarely, a beta particle can be positively charged (positron). The emission of an electron entails the change of a neutron into a proton within the nucleus. The emission of a positron is similarly associated with the change of a proton into a neutron.
See also: Radioactivity.