Definition
Interlanguage is the term for a dynamic linguistic system that has been developed by a learner of a second language (or L2) who has not become fully proficient yet but is approximating the target language: preserving some features of their first language (or L1), or overgeneralizing target language rules in speaking or writing the target language and creating innovations.
Interlanguage work is a vibrant microcosm of linguistics. It is possible to apply an interlanguage perspective to learners’ underlying knowledge of the target language sound system (interlanguage phonology), grammar (morphology and syntax), vocabulary (lexicon), and language-use norms found among learners (interlanguage pragmatics).
Fossilization
Fossilization is the ‘freezing’ of the transition between the native language and the target language. Reasons for this phenomenon may be due to complacency or inability to overcome the obstacles to acquiring native proficiency in the target language. This phenomenon occurs often in adult language learners. Fossilization occurs when a L2 learner is capable of conveying message with current language knowledge, therefore the need to correct the form/structure is not required. Thus, the learner fossilizes the form instead of correcting it.