Definition:Occurrence (Formal Systems)

Definition
Let $\mathcal F$ be a formal language.

Let $S$ be a string in the alphabet of $\mathcal F$.

Let $\mathbf W$ be a well-formed word of $\mathcal F$.

Each place where $S$ appears in $\mathbf W$ is called an occurrence of $S$ in $\mathbf W$.