Q: What is “spin” in particle physics? Why is it different from just ordinary rotation?
Physicist: “Spin” or sometimes “nuclear spin” or “intrinsic spin” is the quantum version of angular momentum. Unlike regular angular momentum, spin has nothing to do with actual spinning. It seems strange to abandon the idea of rotation when talking about angular momentum, but there it is. Instead, a particle’s angular momentum is just another property that it has, like charge or mass. “m” is the amount of angular momentum (well… “ ” is), and is defined as the state that has that amount of angular momentum.