- any hadron, or strongly interacting particle, other than a baryon. Mesons are bosons, having spins of 0, 1, 2, …, and, unlike baryons, do not obey a conservation law.
- any of a group of elementary particles, such as a pion or kaon, that usually has a rest mass between those of an electron and a proton, and an integral spin. They are responsible for the force between nucleons in the atomic nucleus
