property that is characteristic of families of related subatomic particles differing principally in the values of their electric charge. The families of similar particles are known as isospin multiplets: two-particle families are called doublets, three-particle families are called triplets, and so on.

The component particles of atomic nuclei, the neutron and proton, form an isospin doublet, since they appear to differ in nothing but electric charge and subsidiary properties. They are commonly thought of as different versions, or charge states, of the same object, called a nucleon. The isospin of a nucleon has a value of one-half. Isospin values are found by subtracting one from the number of members in its multiplet and then dividing by two.

The main importance of isospin in physics is that, when particles collide or decay under the influence of the strong nuclear force (strong force), their isospin is conserved. That is to say, even as the particles rearrange themselves or change into new particles, their total isospin value, computed in a specified way from the individual values, remains constant. Rules like this and others applicable to isospin (known as isospin selection rules) help physicists to consolidate their understanding of fundamental laws.