Definition:Classes of WFFs/Plain Sentence

Definition
Let $\mathcal L_1$ be the formal language of predicate calculus. A WFF is said to be a plain sentence iff it is both plain and a sentence.

That is, if it contains free variables nor parameters.

Thus, plain sentences are those WFFs which are in $SENT \left({\mathcal P, \mathcal F, \varnothing}\right)$.