An arity of two is also known as binary. A binary relationship, the most common on a traditional data model diagram, involves two entities. The Binary Relationship figure, a UML class diagram, shows that both Student and Course are entities participating in a binary relationship.
An arity of three, known as ternary, is a relationship that includes three entities. An example in fact-based modeling (object -role notation) appears in the Ternary Relationship figure. Here Student can register for a particular Course in a given Semester.
DAMA Data Management Body of Knowledge 2nd Edition, 2017, Print.