An interrelationship is the connection itself — the tie that lets two things push and pull on one another. It is heavier than relationship, which can be any kind of link, even a weak or one-way one. You will see it in science, social studies, and careful essays, naming the bond between ideas, systems, or groups. People do not say it much at the dinner table; they say link, or how the two go together. Though actually, some writers pile on the extra length when relationship would have been enough, so the word can sound serious without adding much. When it is used with care, that inter- still matters: both sides are in play.