Idiosyncrasy

Idiosyncrasy is a property of words or phrases which cannot be derived by the rules of a language. Words can be idiosyncratic in a variety of ways: (a) semantically (by having some unpredictable aspect to their meaning), (b) phonologically (by being an exception to a phonological rule), or (c) morphologically (by being an exception to a word formation rule).

Example
an idiosyncratic property of the English verb derive is that it does not have a nominal counterpart formed with the suffix -al (*derival)

Link
Utrecht Lexicon of Linguistics