Kurtosis

Kurtosis is often used to measure the heaviness of a distribution’s tail:

$$\text{Kurt}_X=\frac{\mathbb{E}[(X-\mu_X)^4]}{\sigma_X^4}=:\frac{\mu_4}{\sigma^4}$$

If a distribution’s kurtosis is $\gt 3$, then it’s called heavy tailed, otherwise $\lt 3$ light tailed.