Definition:Well-Formed Part

Definition
Let $$\mathcal L$$ be a formal language with alphabet $$\mathcal A$$.

Let $$\mathbf A$$ be a well-formed word of $$\mathcal L$$.

Let $$\mathbf B$$ be a substring of $$\mathbf A$$.

Then $$\mathbf B$$ is a well-formed part of $$\mathbf A$$ iff $$\mathbf B$$ is a well-formed word of $$\mathcal L$$.

In the contexts of propositional calculus and predicate calculus, where the term used is well-formed formula, the term subformula can often be seen.