\begin{definition}[Definition:Infix Notation/Binary Relation]
Let $\RR \subseteq S \times T$ be a binary relation.
When $\tuple {s, t} \in \RR$, we can write either:
:$\map \RR {s, t}$
or
:$s \mathrel \RR t$
The notation $s \mathrel \RR t$ is known as '''infix notation'''.
\end{definition}