A relation between two sets is a collection of ordered pairs containing one object from each set. If the object x is from the first set and the object y is from the second set, then the objects are said to be related if the ordered pair (x,y) is in the relation. A function is a type of relation.
More formally, a relation is defined as a subset of A×B. The domain of a relation is the set of elements in A that appear in the first coordinates of some ordered pairs, and the image or range is the set of elements in B that appear in the second coordinates of some ordered pairs.
