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.