A elementary particle is one that has no internal structure; that is, no experiment has observed constituent particles that combine to make it up. It has proven true historically that particles considered "elementary" in one epoch, the atom for example, are later found to be composite. Another example are the mesons, once thought to be the quanta whose exchange between neutrons and protons embodied the nuclear force, but now considered to made up of quarks.
In the Standard Model elementary particles fall into different groups: the group of "particles" themselves, which fall under the classifications of leptons and quarks, and the particles that mediate the interactions between them, the force carriers or field quanta, that fall under the categories of photons, weak bosons, and gluons.
- Martinus Veltman (2003). Facts and Mysteries in Elementary Particle Physics. World Scientific, p. 13. ISBN 981238149X.