Definition:Well-Formed Formula

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