Diacritic feature

Diacritic feature is a formal expression of unpredictable information about words in their lexical entry.

Example
many non-native English verbs may undergo -ation and/or -al affixation (recite: recital: recitation). However, the verbs arrive and derive do not allow the derivation of *arrivation and *derival, respectively. Halle (1973) accounts for these accidental gaps by assigning the diacritic feature [-lexical insertion] to these forms. Other widely used diacritic features are [+/- latinate] or [+/- native] (e.g. Aronoff 1976). Another term is exception feature.

Link
Utrecht Lexicon of Linguistics