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).