Definition:Well-Formed Part

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

Let $\mathbf A$ be a well-formed formula of $\mathcal F$.

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

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

Also known as
In sources where WFFs are referred to as formulas, the term subformula can often be seen.

Likewise, in sources where WFFs are called expressions, subexpression is the name of choice.