alpha particle

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

• noun A positively charged subatomic particle, composed of two protons and two neutrons, identical with the nucleus of the helium isotope He-4.

from the GNU version of the Collaborative International Dictionary of English.

• (Physics & Chem.) a particle emitted at high velocity from certain radioactive substances. It is identical to the helium nucleus, consisting of two protons and two neutrons. Rays of such particles are called alpha rays.

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

• noun A positively charged nucleus of a helium-4 atom (consisting of two protons and two neutrons), emitted as a consequence of radioactivity; α-particle

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

• noun a positively charged particle that is the nucleus of the helium atom; emitted from natural or radioactive isotopes