- Any constituent part of an atom, such as the proton, neutron and electron, or of a possible atom, like the positron, as well as muons, mesons and other components of atomic nuclei. These are sometimes called elementary particles although some appear more fundamental than others. Photons are the elementary particles of electromagnetic radiation.
- A very small or infinitesimal portion of matter, such as a molecule, atom, proton, or quark.
- 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 elementary particle.
- A comparatively small subdivision of matter, such as magnetic particles or dust.
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