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In CycL, a well-formed formula or WFF (pronounced “woof”, same vowel as “foot”) is a logical formula that is syntactically and semantically valid.

Well-formedness is different in CycL than in other logical representation languages. Most other representation languages have a notion of well-formedness that only implements syntactic checks. In such languages, a formula is either part of the language (well-formed) or not part of the language. See Wikipedia:Well-formed formula for more on this definition of well-formedness.

In CycL, a formula can be syntactically correct while still violating semantic constraints. A formula that only violates semantic constraints, while not well-formed, is still considered part of the language. Such a formula is definitively false, but not ungrammatical.

In Cyc, the well-formedness of a formula is checked by the WFF Checker.