You want to know what g-2 is? Well, g is the gyromagnetic ratio for the electron, which is the actual magnetic moment divided by what it would be if it were a classical ball of spinning charge with the same angular momentum. Two is the value it would have for the Dirac equation (relativistic quantum mechanics, but without radiative corrections). g-2 is the correction due to quantum electrodynamics, and is called the anomalous magnetic moment of the electron.
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Statistical Mechanics: Entropy, Order Parameters, and Complexity, now available at Oxford University Press (USA, Europe).