Binary Relation
A binary relation \(R\) over sets \(X\) and \(Y\) is a subset of the cartesian product of \(X \times Y\). Where \(X\) is the domain, or set of departure of \(R\), and \(Y\) is the codomain, or set of destination of \(R\). An element \(x \in X\) is related to \(y \in Y\), iff the ordered pair \((x, y)\) is found within the (above) subset.