a relation , it's just as simple as that
a relation that always takes exactly two arguments
a set of ordered pair s"
a set of ordered pairs
a special case of a k -ary relation , that is, a set of k -tuples where the j th component of each k -tuple is taken from the j th domain X j of the relation
a subset of the Cartesian product of two sets and
a subset of the cross product of two sets
a two-place relation
In mathematics, a binary relation (or a dyadic relation) is an arbitrary association of elements of one set with elements of another (perhaps the same) set.