Spin (physics) discrete angular momenta despite having no orbital angular momentum. In some ways, spin is like a vector quantity; it has a definite magnitude, and it has a "direction" (but quantization makes this "direction" different from the direction of an ordinary vector). All elementary particles of a given kind have the same magnitude of spin angular momentum, which is indicated by assigning the particle a spin quantum number. The SI unit of spin is the (N·m·s) or (kg·m·s), just as with classical angular momentum. In practice, spin is given as a dimensionless spin quantum number by dividing the spin angular momentum
Physical quantity generally well understood (everyone understands what is meant by "the frequency of a periodic phenomenon", or "the resistance of an electric wire"). The term "physical quantity" does not imply a physically "invariant quantity". "Length" for example is a "physical quantity", yet it is variant under coordinate change in special and general relativity. The notion of physical quantities is so basic and intuitive in the realm of science, that it does not need to be explicitly "spelled out" or even "mentioned". It is universally understood that scientists will (more often than not) deal with quantitative data, as opposed to qualitative data.