The Collaborative International Dictionary of English v.0.48:

Covariant \Co*va"ri*ant\ (k?-v?"r?-ant), n. (Higher Alg.)
   A function involving the coefficients and the variables of a
   quantic, and such that when the quantic is lineally
   transformed the same function of the new variables and
   coefficients shall be equal to the old function multiplied by
   a factor. An invariant is a like function involving only the
   coefficients of the quantic.
   [1913 Webster]

WordNet (r) 3.0 (2006):

covariant
    adj 1: changing so that interrelations with another variable
           quantity or set of quantities remain unchanged