\begin{definition}[Definition:Instance]
Let $\mathbf C$ be a plain WFF in the language of predicate logic.
Let $x_1, x_2, \ldots, x_n$ be the free variables of $\mathbf C$.
Let $\MM$ be a structure for predicate logic of type $\PP$ whose universe set is $M$.
Then an '''instance of $\mathbf C$ in $M$''' is the sentence with parameters from $M$ formed by choosing $a_1, a_2, \ldots, a_n \in M$ and replacing all free occurrences of $x_k$ in $\mathbf C$ by $a_k$ for $k = 1, \ldots, n$.
The resulting sentence is denoted:
:$\map {\mathbf C} {x_1, \ldots, x_n \,//\, a_1, \ldots, a_n}$
Thus $\map {\mathbf C} {x_1, \ldots, x_n \,//\, a_1, \ldots, a_n} \in \map {SENT} {\PP, M}$.
If $\mathbf C$ is a plain sentence, then no parameters are needed, and $\mathbf C$ is already an instance of itself.
\end{definition}