According to the following definition:
q-complements/substitutes (q is for quantity). Input x and y are complements if the marginal product of x is increasing in quantity of y. Input x and y are substitutes if the marginal product of x is decreasing in quantity of y.
Complements are sometimes called "imperfect substitute". See, for instance,this paper (Autor et al. 2003), page 1286-1287.