The term for a particle’s intrinsic angular momentum is spin. Unlike orbital angular momentum, which comes from motion through space around a point, spin is an inherent quantum property carried by particles such as electrons, protons, neutrons, and photons.
Spin is quantized rather than continuously variable. A particle is assigned a spin quantum number, often written s, and the possible results of measuring a component of spin occur in discrete steps. For an electron, the spin quantum number is 1/2, so a measurement along a chosen axis gives one of two projections, commonly called spin-up or spin-down.
The Stern–Gerlach experiment, first performed with silver atoms in 1922, provided striking evidence for discrete angular-momentum states. Spin later became central to the Pauli exclusion principle, atomic structure, magnetism, and the classification of particles as fermions or bosons.
Spin does not mean that a tiny solid sphere is literally rotating. That classical picture cannot account for the measured properties of quantum particles. A related mix-up is parity: parity describes how a state behaves under spatial reflection, whereas spin describes intrinsic angular momentum.