beta particle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An electron or positron, especially one emitted at high energy in the decay of certain radioactive nuclei.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics An energetic electron or positron produced as the result of a nuclear reaction or nuclear decay.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a high-speed electron or positron emitted in the decay of a radioactive isotope