\begin{definition}[Definition:Many-to-One Relation/Defined/Element]
Let $f \subseteq S \times T$ be a many-to-one relation.
<section notitle="true" name="tc-defn">
Let $s \in S$.
Then $f$ is '''defined at $s$''' {{iff}} $s \in \Dom f$, the domain of $f$.
</section>
\end{definition}