Photons

Massless packet of energy, which behaves like both a wave and a particle. In the duality of mass/energy this is considered a “particle” of radiation.

Einstein’s generalization of the concept of the quantum introduced by Max Planck.

See also: Eddington Limit, Stimulated Emission, Wave Particle Duality.

Further reading:
• Advanced Physics for You, Keith Johnson, Simmone Hewett, Sue Holt, John Miller, Nelson Thornes