What is covariance?

covariance — A statistical measure used in computing the correlation coefficient between two variables; the covariance is the mean of (x- x(bar))(y-y(bar)) over all pairs of values for the variables x and y, where x(bar) is the mean of the x values and y(bar) is the mean of the y values. A statistical value measuring the simultaneous deviations of x and y variables from their means.