Quantum electrodynamics, or QED, is a quantum field theory of electromagnetism. QED describes all phenomena exhibited by charged point particles, such as electrons and positrons, and the particles of light (photons), interacting by electromagnetism. This theory includes classical electrodynamics in the limit of large fields, but also explains purely quantum phenomena such as the structure of atoms and molecules, the creation of particles by an electromagnetic field, and the anomalous magnetic moment of the electron. (The latter prediction has been experimentally confirmed to 11 decimal digits.)
QED was the first quantum field theory in which the difficulties of building a consistent, fully quantum description of fields and creation and annihilation of quantum particles were satisfactorily resolved. Sin-Itiro Tomonaga, Julian Schwinger, and Richard Feynman received the 1965 Nobel Prize in Physics for its development.
Pure QED in 4 dimensions with n particles is reserved for the quantum field theory with the Lagrangian
See sign convention.