Definition:Occurrence (Predicate Logic)

Definition
Let $\mathbf A$ be a WFF of predicate logic.

Let $S$ be a string in the alphabet of predicate logic.

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

Note that $S$ may consist of a single symbol, but may not be null.

Also see

 * Occurrence (Formal Systems)