I have, since 1870, written much about the logic of relations. In those writings, I have usually restricted the terms “relations” and “relationships” to existential relations and relationships. By a relationship I understand the conception of a fact about a set of things abstracted from the representation of the things themselves or, in other words, a predicate which requires more than one subject to complete a proposition, or conception of a fact.
http://www.commens.org/dictionary/entry/quote-fourth-curiosity-10