noun. A statistical measure of the tendency of two random variables to vary in the same direction (called positive covariance) or in an opposite direction (called negative covariance) over many observations. Covariance is equal to the summed products of the deviations of corresponding values of the two variables from their respective means.

noun. a statistical measure of the relationship of two variables, formed by multiplying the difference of each variable from its mean, both variables being measured at the same time, and averaging all such products.

noun. A statistical measure defined as given two real-valued random variables X and Y, with expected values and .

noun. The conversion of data types from wider to narrower in certain situations.

noun. (statistics) the mean value of the product of the deviations of two variates from their respective means

"If so, this would explain the reference to a "correlation matrix" in their article and could also be reconciled with the use of the term covariance matrix in the email."

covariance matrix  

variance  

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