In physics a nucleon is a collective name for two baryons: the neutron an the proton. They are constituents of the atomic nucleus and until the 1960s were thought to be elementary particles. In those days their interactions (now called internucleon interactions) defined strong interactions. Now they are known to be composite particles, made of quarks and gluons. Understanding the nucleons' properties is one of the major goals of quantum chromodynamics, the modern theory of strong interactions.