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Fundamental characteristic property of particles which together with orbital angular momentum acts as the generator of rotations and which doesn't have a classical equivalent but is sometimes compared to and contrasted with classical intrinsic angular momentum.

The spin is an angular momentum in the sense that it determines the number of allowed orientations of the intrinsic magnetic moment, but is independent of the motion of the particle in space.

Fundamental particles are infinitesimal points of zero size, so it’s hard to imagine what it even means for them to rotate. We can, however, measure their angular momentum, and it seems to always come in discrete multiples of $~\frac{1}{2 }~$like $~-~\frac{1}{2 }~$, $~0~$, $~+~\frac{1}{2 }~$, $~+1~$, and nothing in between. This is a quantum mechanical effect, and each particle has an intrinsic or built-in angular momentum called spin.

Particles of matter, such as electrons and quarks, all have spin $~\frac{1}{2 }~$ (called fermions), whereas particles of force or energy have integer spins: $~0,~ 1,~$ and $~2~$ (called bosons). Photons, which are particles of light, can have spin $~-1~$ or $~+1~$, a phenomenon that is known to photographers as circular polarization.