Source:http://linkedlifedata.com/resource/umls/id/C0086805
MSH: Discrete concentrations of energy, apparently massless elementary particles, that move at the speed of light. They are the unit or quantum of electromagnetic radiation. Photons are emitted when electrons move from one energy state to another. (From Hawley's Condensed Chemical Dictionary, 11th ed),NCI: A single unit of electromagnetic radiation, generally considered to be a discrete particle having no mass or charge.