What is the X in X-mas?
A vector quantity of quantum systems (especially subatomic particles) that is conserved in strong interactions and is used to characterize particles that are affected equally by the strong force but have other dissimilar properties, such as different electric charges. For example, protons and neutrons, which are indistinguishable with respect to strong force interactions, have the same isospin. Also called isotopic spin. See also baryon number, strangeness.