Fisher Kurtosis

$$\gamma_2 \equiv b_2 \equiv {\mu_4\over {\mu_2}^2} - 3 = {\mu_4\over \sigma^4} - 3,$$

where $\mu_i$ is the $i$th Moment about the Mean and $\sigma=\sqrt{\mu_2}$ is the Standard Deviation.

See also Fisher Skewness, Kurtosis, Pearson Kurtosis