The term ‘interlanguage’ was coined by the American linguist, Larry Selinker, in recognition of the fact that L2 learners construct a linguistic system that draws, in part, on the learner’s L1 but is also different from it and also from the target language.
A learner’s interlanguage is, therefore, a unique linguistic system. The concept of interlanguage involves some premises about L2 acquisition:
The learner constructs a system of abstract linguistic rules that underlies comprehension and production of the L2. This system is viewed as a ‘mental grammar’ and is referred to as ‘interlanguage.’
The learner’s grammar is open to influence from the outside (input) and inside (internal processing). Some of the internal processing characteristics are known as omission, overgeneralization, and transfer.
The learner’s grammar is transitional. It changes from time to time (adding rules, deleting rules, restructuring the whole system). The natural product is an interlanguage continuum. That is, learners construct a series of mental grammars or interlanguages as they gradually increase the complexity of their L2 knowledge.
The different kinds of errors learners produce reflect different learning strategies. For example, omission errors may suggest that learners are in some way simplifying the learning task by ignoring grammatical features that they are not yet ready to process. Overgeneralization and transfer errors can also be seen as evidence of learning strategies.
The learner’s grammar is likely to fossilize. Selinker suggested that only about five per cent of learners go on to develop the same mental grammar as native speakers. The majority stop some way short. The prevalence of backsliding is typical of fossilized learners. Fossilization does not occur in L1 acquisition and thus is unique to L2 grammars.
(From: Ellis, R (2001) Second Language Acquisition. OUP.)