relation - an abstraction belonging to or characteristic of two entities or parts together

complementarity - the interrelation of reciprocity whereby one thing supplements or depends on the other; "the complementarity of the sexes"

correlation, correlativity - a reciprocal relation between two or more things

interdependence, interdependency, mutuality - a reciprocal relation between interdependent entities (objects or individuals or groups)

mutuality, mutualness - a reciprocality of sentiments; "the mutuality of their affection was obvious"

reciprocal - something (a term or expression or concept) that has a reciprocal relation to something else; "risk is the reciprocal of safety"