positively charged particles that consist of two protons and two neutrons (i.e., the nucleus of a helium atom), that may be emitted during radioactive decay.
the nucleus of a helium atom, consisting of two protons and two neutrons
These are particles possessing a positive charge, and are emited from radioactive elements. The particles are actually helium nuclei, ie. helium atoms that had lost their outer electrons. The study of the alpha particle played a major part in the development of modern atomic theory. Rutherford bombarded other elements with alpha particles to produce nuclear disintegration.