Wiktionary
well-formed formula
n. (context logic English) A statement that is expressed in a valid, syntactically correct, manner.
Wikipedia
Well-formed formula
In mathematical logic, a well-formed formula, abbreviated wff, often simply formula, is a finite sequence of symbols from a given alphabet that is part of a formal language. A formal language can be identified with the set of formulas in the language.
A formula is a syntactic object that can be given a semantic meaning by means of an interpretation. Two key uses of formulas are in propositional logic and predicate logic.