An interlanguage is an emerging 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. An interlanguage is idiosyncratically based on the learners' experiences with the L2. It can fossilize, or cease developing, in any of its developmental stages. The interlanguage rules are claimed to be shaped by several factors, including: L1 transfer, transfer of training, strategies of L2 learning (e.g. simplification), strategies of L2 communication (or communication strategies like circumlocution), and overgeneralization of the target language patterns.

Interlanguage is based on the theory that there is a "psychological structure latent in the brain" which is activated when one attempts to learn a second language. Interlanguage theory is usually credited to Larry Selinker but others such as Uriel Weinreich have claimed to have formulated the basic concept before Selinker's 1972 paper. Selinker noted that in a given situation the utterances produced by the learner are different from those native speakers would produce had they attempted to convey the same meaning. This comparison reveals a separate linguistic system. This system can be observed when studying the utterances of the learner who attempts to produce meaning in using the target language; it is not seen when that same learner does form-focused tasks, such as oral drills in a classroom. Interlanguage can be observed to be variable across different contexts; for example, it may be more accurate, complex and fluent in one discourse domain than in another (Tarone, 1979; Selinker & Douglas, 1985).

To study the psychological processes involved one should compare the interlanguage utterances of the learner with two things:

  1. Utterances in the native language to convey the same message produced by the learner
  2. Utterances in the target language to convey the same message, produced by a native speaker of that language.

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).

By describing the ways in which learner language conforms to universal linguistic norms, interlanguage research has contributed greatly to our understanding of linguistic universals in SLA. See below, under "linguistic universals".

Read more about Interlanguage:  Definition and Relation To Other Types of Language, Variability, Developmental Patterns, Linguistic Universals