\begin{definition}[Definition:Formal Semantics/Valid]
Let $\LL$ be a formal language.
Part of specifying a formal semantics $\mathscr M$ for $\LL$ is to define a notion of '''validity'''.
Concretely, a precise meaning needs to be assigned to the phrase:
:"The $\LL$-WFF $\phi$ is '''valid''' in the $\mathscr M$-structure $\MM$."
It can be expressed symbolically as:
:$\MM \models_{\mathscr M} \phi$
\end{definition}