In linguistic morphology, fossilization refers to two close notions.
One is preserving of ancient linguistic features which have lost their grammatical functions in language.
Another is loss of productivity of a grammatical paradigm (e.g. of an affix), which still remains in use in some words.
[1] The term interlanguage fossilization refers to common types of errors made by most adult second-language learners, differing from the idiomatic usage of native-language learners.
[3] These errors occur regardless of exposure to the language or education level.