An ELT Glossary : Fossilization

Definition : Fossilization is process which sometimes occurs in which incorrect linguistic features gradually become a permanent part of the way a learner speaks or writes the language. 

Example :  A C2 level speaker may still leave off the 3rd person "s", despite using other more complex structures accurately and at a generally "expert" level. 

Associated with the work of Selinker in the 1970s.

Further reading 
Routledge Encyclopedia of Language Teaching and Learning

Ellis, R. Second Language Acquisition, OUP

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