An Alpha particle consists of two neutrons and two protons bound as a single particle that is emitted from the nucleus of certain radioactive isotopes in the process of decay.

Other definition:
An alpha particle is a positively charged particle made up of two neutrons and two protons emitted by certain radioactive nuclei. Alpha particles can be stopped by thin layers of light materials, such as a sheet of paper, and pose no direct or external radiation threat. However, they can pose a serious health threat if ingested or inhaled. For a better Understanding Radiation see also Alpha Particles.