1) A helium nucleus.
2) Consists of the nucleus of an atom of helium (two protons and two neutrons) and is sometimes emitted from the nucleus of some atoms as part of those atoms' radioactive decay. Alpha particles have an energy range of 4-8 MeV. This energy is dissipated in only a few centimeters of air or less than 0.005mm of aluminum (Al).
3) A cluster of 2 protons and 2 neutrons emitted from a nucleus in one type of radioactivity.