beta particle

electron in a beta ray, high-speed electron or positron, electron or positron ejected from the nucleus of an atom during radioactive decay

electrons or positrons ejected from an atom's nucleus during radioactive decay

Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay. They are designated by the Greek letter beta (β). There are two forms of beta decay, β− and β+, which respectively give rise to the electron and the positron.
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Beta particle n. A high-speed electron or positron, especially one emitted from the nucleus of an atom in radioactive decay. [more]Beta particle - Community and Resources

Noun
1. a high-speed electron or positron emitted in the decay of a radioactive isotope
(hypernym) particle

A high speed electron emitted in a form of radioactive disintegration from an atomic nucleus which has a surfeit of electric charge and energy. The mechanism involves the conversion of one type of nucleon  into another and the creation of an electron ( no  electrons can exist in the nucleus (the so-called Klein paradox  )). The beta particle is emitted at high speed at any one energy up to a maximum which is unique to the radio-nuclide. Likewise the half life is characteristic. Energy is conserved by the simultaneous emission of a neutrino with very little (or no mass) that travels close to (or at the speed of light). An example of the equation of 'nuclear chemistry' that illustrates negative beta decay is the deacy of tritium with a 13 year half-life 1H3 -> 2He3 + -e + antineutrino
If the electron emitted is the antimatter counterpart or positron then there must first be an excess of internal energy in the nucleus greater than the rest mass energy of two electrons (i.e. 2 m c2). An 'ordinary' matter neutrino shares energy with the positive electron. For example: 8O15 + (2 m c2) -> 7N15 + +e + neutrino