A relation is how the elements in the domain relate to the elements in the codomain.

Domain of the relation - the "subject" of the relation.

Co-domain of the relation - the "object" of the relation.

Elements of the Relation:
Suppose the domain is {Matt, Ethan, Jed} and the co-domain is {Cat, Dog, Lion} The ordered pairs of the domain and co-domain are (Matt, Cat) , (Ethan, Dog) , and (Jed, Lion) We call these the elements of the relation.