A quantum of electromagnetic energy. Like electrons, photons appear as both waves and particles at the same time. Quite often, a photon is said to be a "particle of light;" however, radio transmission, X-rays and gamma rays are also made up of particles. Although they may not always be called photons, they are the same phenomena at different frequencies.
The energy of an individual photon is proportional to its frequency, which is why a single photon of light has more energy than a photon in the radio spectrum below it. A single light photon can cause a neuron in your retina to fire or convert silver iodide to silver and iodine on photographic film. However, a single radio photon is nearly impossible to detect, and all by itself, is not doing anything that we want to measure. See photoelectric
and wave-particle duality