A relationship set is a set of relationships of the similar type.
For example, let consider the relationship among two entities sets course and student. Collection of all the instances of relationship opts forms a relationship set known as relationship type.
Degree
The degree of a relationship type is the number of participating entity types.
The relationship among two entities is known as binary relationship. A relationship among three entities is known as ternary relationship.
Likewise relationship between n entities is called n-ry relationship.