A hadron is a particle that interacts with the strong nuclear force. The term hadron was first used in 1964 by Lev B. Okun to refer to particles that participate in the strong interaction. The four known hadrons are the proton, neutron, pion, and antiproton.
The strong nuclear force is one of the four fundamental forces of nature, along with gravity, electromagnetism, and the weak nuclear force. It is responsible for holding together the nuclei of atoms and is about 100 times stronger than the electromagnetic force. The strong nuclear force is mediated by particles called gluons.
Gluons are massless particles that carry the color charge of quantum chromodynamics (QCD). They bind quarks together to form hadrons such as protons and neutrons. Gluons also mediate interactions between hadrons via the exchange of virtual mesons. Mesons are bosonic particles that carry both color and flavor charges.
The most important property of hadrons is their mass. The masses of Hadrons arise from two different sources: the kinetic energy of their constituent quarks and gluons, and their binding energy (the energy required to hold them together). The vast majority of a hadron’s mass comes from its binding energy.
The properties of hadrons can be understood using quantum chromodynamics (QCD), which is the theory of strong interactions. QCD describes how quarks and gluons interact via the exchange of virtual mesons. This interaction gives rise to the Strong Force, which holds Hadrons together. In addition, QCD predicts that Hadrons have spin 1/2 due to their fermionic constituents (quarks).