A type of diagram used in perturbation theory calculations of the interactions between the particles in a system. Such diagrams were invented by Richard Feynman (1918–88) in the context of quantum electrodynamics, although similar diagrams had been used earlier to describe the interactions between molecules in a real gas. Feynman diagrams have been used very extensively to describe the interactions between electrons in atoms, molecules, and solids and between the particles in a system in statistical mechanics.