covariance / covariants [ko:ˈvɛriəns]
covariance – n. – Math the arithmetic mean or the expected value of the product of the deviations of corresponding values from their respective mean values
covariants – n. pl. & adj. – n. – a variant that changes leaving interrelations with another variant (or variants) unchanged; adj. – changing along with something else so as to preserve certain mathematical interrelations unchanged