mutual relationship; interrelationship; interaction
intercommunication (usu. in telecoms contexts)
mutual assured destruction
mutual use of company facilities (e.g. rail tracks for through service, airport facilities, code-sharing, etc.)
cross connection; interconnection; internetworking
interconnection; interoperability
law of quadratic reciprocity
transinformation (content); transferred information; transmitted information; mutual information
mutual visitation; reciprocal visits
interchangeability; interoperability