In contrast, a form that violates some grammar rule is ill-formed and does not constitute part of the language.
For example, the nonce word wug coined by Jean Berko Gleason is phonologically well-formed, so informants are able to pluralize it regularly.
[1] A word, phrase, clause, or utterance may be grammatically well-formed, meaning it obeys the rules of morphology and syntax.
A native speaker may judge a word, phrase or pronunciation as "not quite right" or "almost there," rather than dismissing it as completely unacceptable or fully accepting it as well-formed.
Some generative linguists think that ill-formedness might be strictly additive, thus trying to figure out universal constraints by acquiring scalar grammaticality judgments from informants.