From Wikipedia:

This term was derived fromisotopic spin, a confusing term to which nuclear physicists preferisobaric 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. A close examination of this symmetry, historically, led directly to the discovery and understanding of quarks and of the development ofYang–Mills theory.

G. t'Hooft, in the SciAm article I linked earlier, states that isospin symmetry is continuous. This means for instance that proton/neutron isospin can

*describe*a particle in a superposition of proton + neutron. So what about the electric charge of +1 in the case of such a superposition?

Or is it the case that, even though the symmetry is continuous, physically it doesn't exist?