An Alpha Particle is a Particle emitted during a nuclear reaction, such as Radioactive Decay. It specifically consists of two protons and two neutrons, i.e., it is a Helium nucleus. This is in contrast to a Beta Particle, similarly emitted, but consisting of an electron or positron.
Collisions of alpha or beta particles with a nucleus can also create nuclear reactions, e.g., Alpha Capture.
Cosmic Rays include alpha and beta particles.