iso·spin | \ ˈī-sə-ˌspin 
\
\Definition of isospin
: a quantum characteristic of a group of closely related subatomic particles (such as a proton and a neutron) handled mathematically like ordinary spin with the possible orientations in a hypothetical space specifying the number of particles of differing electric charge comprising the group
— called also isotopic spin
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Time Traveler for isospin
The first known use of isospin was in 1961
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