Covariance
The variance is a measure of the variability or dispersion in a variable or data set.
A measure of the variability of one variable (or data set) in relation to another variable (or data set), is the covariance.
The variability of one variable (or data set) {X} in relation to another variable (or data set) {Y} would depend upon three factors:
Hence, if the variable X is the same as the variable Y, the covariance between the two reduces to the variance of either.
In other words, the variance is a special case of covariance.