(quantum of light, i.e., light as a particle)
A photon is a particle of light (electromagnetic radiation), i.e., what you call
such a particle if light is considered a stream of particles. It
is the quantum of light, the minimum possible amount.
The photon theory, i.e., the modern particle theory of light,
is that the minimum amount of light depends upon the light's
frequency, being the light-energy proportional to it.
Dating from prior to photon theory, there has been a very successful
theory that EMR consists of waves, as summarized by Maxwell's equations.
Photon theory coexists because it is successful in modeling
some light/matter interactions that the wave theory does not.
Photons are elementary particles (a type of boson) according to
current particle physics, interacting with other particles. They
naturally travel "the speed of light". As implied above, each
photon has an assigned frequency and carries an amount of energy
proportional to it.
Phenomena successfully modeled with photon theory includes the
interaction of light with atoms and electrons, such as absorption,
emission and scattering. The first such instance for
photon theory was photoionization, i.e., that ionization by
incoming EMR does not simply depend upon the total light energy
striking the atoms, but more basically on the light's frequency, which
Einstein explained as suggesting only those quanta of light
carrying sufficient energy to free an electron from the atom were
successful ionizing such an atom.
baryon acoustic oscillations (BAO)
cosmic microwave background (CMB)
cosmic rays (CR)
Cherenkov Telescope Array (CTA)
dark matter annihilation
de Broglie wavelength
epoch of reionization (EOR)
High-altitude Water Cherenkov Observatory (HAWC)
mean free path
optical depth (τ)
Planck constant (h)
Poynting vector (S)
quantum mechanics (QM)
equation of radiative transfer (RTE)
spectral energy distribution (SED)
superluminous supernova (SLSN)
spectral power distribution (SPD)
synchrotron self-Compton (SSC)
state of excitation
Sunyaev-Zel'dovich effect (SZ Effect)
thermodynamic equilibrium (TE)
ultra-high-energy gamma rays (UHEGR)
very-high-energy gamma rays (VHEGR)
vegetation red edge (VRE)