- noun a particle that is smaller than an atom or forms part of an atom, e.g. an electron.
- Any of the fundamental constituents of all matter in the universe. Elementary particles include protons, neutrons, electrons, photons, and quarks. Categories of such particles include leptons and hadrons. An electron, for instance, is a type of lepton. When an elementary particle is defined as having no internal structure, particles such as protons and neutrons are excluded, as these are composed of quarks. The four fundamental forces of nature are carried by elementary particles. For example, photons carry the electromagnetic force. Also called fundamental particle, or particle (2).