1 What is Spin? Pradip Kumar Chatterjee* The Indian Physical Society 2A & B Raja Subodh Chandra Mullick Road, Kolkata-700032, India, and Calcutta Mathematical Society AE-374, Sector-I, Kolkata -700064, India. ABSTRACT Minimal imaginary momenta of quantum particles available in non-unitary quantum mechanics generate spin of the particle. The D0-branes of a universal constant mass comprise the lowest vibration mode of quantum vacuum. Spin of a particle is just orbital angular momentum of D0-brane(s) of mass equal to mass gap circling along a loop of imaginary radius. For three distinct directions of spin component, each orbit picks up a distinct unit of pure quaternion basis. The interpretation becomes relatively untangled when each unit of pure quaternions is found related to one of the three Pauli spin matrices. An important revelation is that spin in quantum mechanics requires Lorentz violation. Lorentz-violating energy term feeds the energy required for spinning of the D0-brane. Impossibility of a non-composite spin-0 object existing as a truly elementary particle has been examined. PACs number(s): 03.65.-w, 75.10.Jm, 11.30.Cp, 11.15. *email: pradipstring@gmail.com