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