- Any of the fundamental constituents of all matter in the universe. Fundamental 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 a fundamental 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 elementary particle, or particle (2).