Definition:Infix Notation

Definition
Let $\mathcal R \subseteq S \times T$ be a binary relation.

When $\left({s, t}\right) \in \mathcal R$, we can write either:
 * $\mathcal R \left({s, t}\right)$, or
 * $s \mathcal R t$.

The notation $s \mathcal R t$ is known as infix notation.