Overgeneration

Overgeneration is a property of (word formation) rules which entails that they are able to generate entities which are fully well-formed, but are not attested or used by the native speakers of a language.

Example
if the rule for -al affixation may derive attested forms such as arrival and refusal as well as unattested forms such as derival and describal, we say that the -al rule overgenerates.

Links
Utrecht Lexicon of Linguistics