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.