Alpha particle: A tiny mass of material composed of two protons and two neutrons.

Alpha particles do not travel very far from their radioactive source. They cannot pass through a piece of paper, clothes or even the layer of dead cells which normally protects the skin. Because alpha particles cannot penetrate human skin they are not considered an "external exposure hazard." This means that if the alpha particles stay outside the human body they cannot harm it. However, alpha particle sources located within the body may pose an "internal" health hazard if they are present in great enough quantities.

For instance, the risk from indoor radon gas is due to inhaled alpha particle sources which irradiate lung tissue.

Alpha rays are streams of alpha particles.