In quantum physics, an electron's intrinsic angular momentum is called spin. Electron spin is an intrinsic quantum property, not literal rotation of a tiny solid ball. Its magnitude is associated with spin quantum number 1/2, making the electron a fermion.
A spin-1/2 particle has two possible outcomes when measured along a chosen axis, conventionally labeled spin up and spin down. The measurement values are often written +ħ/2 and −ħ/2. These labels describe quantized results, not a classical arrow that can point in every direction while being observed.
Electron spin also creates a magnetic moment, allowing electrons to interact with magnetic fields. Stern–Gerlach experiments with atoms provided early evidence for space quantization, while later techniques directly explored electron spin behavior. Spin is central to atomic structure, magnetism, magnetic resonance, and technologies such as spin-based electronics.