"Isospin" is short for isotopic spin, which comes from the word isotope. Isotopic spin is a misnomer: by definition isotopes differ by the number of nucleons they contain, but isospin rotations do not change the number of nucleons in a nucleus. Isospin rotations “turn protons into neutrons” or vice versa. The term isobaric (meaning “same number of baryons”) spin replaced isospin for a while, but the shorter name isospin is far more common today.