Correlative
CORRELATIVE, adjective [Latin Con and relativus. See Relate and Relative.] Having a reciprocal relation, so that the existence of one in a certain state depends on the existence of another; as father and son, husband and wife, are correlative terms. The term son is correlative to that of father.
CORRELATIVE, noun That which is opposed to something else in a certain relation. The son is the correlative of his father. Darkness and light are correlatives. Rest is the correlative of motion.