How is covariance related to the variance of a single variable?

In statistics, the covariance between a variable and itself is equivalent to the variance of that single variable. This relationship is expressed by the formula:

$$\text{Cov}(X,X)=\text{Var}(X)$$

While covariance generally measures the joint variability between two different random variables, applying the covariance calculation to a single variable $X$ effectively measures how that variable deviates from its own mean, resulting in its variance.1