Definition:Well-Formed Formula

Definition
Let $\mathcal F$ be a formal language whose alphabet is $\mathcal A$.

A well-formed word is a word in $A$ which can be built by using the rules of formation of the formal grammar of $\mathcal F$.

That is, a word in $A$ is a well-formed word in $\mathcal F$ iff it has a parsing sequence in $\mathcal F$.

In the contexts of propositional calculus and predicate calculus, the term used is usually well-formed formula or WFF (pronounced something like oof or woof, depending on taste).