In physics, and specifically, particle physics, isospin (isotopic spin, isobaric spin) is a symmetry of the strong interaction as it applies to the interactions of the neutron and proton. This term was derived from isotopic spin, but the term isotopic spin is confusing as two isotopes of nucleus have different amount of nucleons, besides, rotations of isospin maintain the number of nucleons. Nuclear physicists prefer isobaric spin, which is more precise in meaning. Isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions of baryons and mesons. Isospin symmetry remains an important concept in particle physics, and a close examination of this symmetry historically led directly to the discovery and understanding of quarks and of the development of Yang-Mills theory.