\begin{definition}[Definition:Pairwise Perpendicular]
Let $S = \set {L_1, L_2, \ldots, L_n}$ be a set of lines in a Euclidean space.
Then the elements of $S$ are '''pairwise perpendicular''' {{iff}}
:$\forall i, j \in \set {1, 2, \ldots, n}, i \ne j:$ $L_i$ and $L_j$ are perpendicular to each other.
\end{definition}