- a positively charged particle consisting of two protons and two neutrons, emitted in radioactive decay or nuclear fission; the nucleus of a helium atom.
- a helium-4 nucleus, containing two neutrons and two protons, emitted during some radioactive transformations
- A positively charged nuclear particle, indistinguishable from a helium atom nucleus and consisting of two protons and two neutrons.
- A positively charged particle that consists of two protons and two neutrons bound together. It is emitted by an atomic nucleus undergoing radioactive decay and is identical to the nucleus of a helium atom. Because of their relatively large mass, alpha particles are the slowest and least penetrating forms of nuclear radiation. They can be stopped by a piece of paper. See more at radioactive decay.