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

The Collaborative International Dictionary of English v.0.48:

covariance \covariance\ n. 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. [WordNet 1.5 +PJC]WordNet (r) 3.0 (2006):

covariance n 1: (statistics) the mean value of the product of the deviations of two variates from their respective means