\begin{definition}[Definition:Rank Function for Relation]
Let $\struct {S, \RR}$ be a relational structure.
Let $\struct {T, \prec}$ be a strictly well-ordered set.
Let $\operatorname {rk}: S \to T$ be a mapping such that:
:$\forall x, y \in S: \paren {x \ne y \text { and } \tuple {x, y} \in \RR} \implies \map {\operatorname {rk} } x \prec \map {\operatorname {rk} } y$
$\operatorname {rk}$ is known as a '''rank function for $\RR$'''.
\end{definition}