\begin{definition}[Definition:Permutation/Ordered Selection]
Let $S$ be a set of $n$ elements.
Let $r \in \N: r \le n$.
An '''$r$-permutation of $S$''' is an '''ordered selection''' of $r$ elements of $S$.
From this definition, it can be seen that a bijection $f: S \to S$ (defined as a mapping) is an '''$n$-permutation'''.
\end{definition}