A term coined by Selinker in 1972 to characterise the learner's developing competence in the target language. 

Early applied linguist Pit Corder was the first to describe learner language as "systematic," akin to a "built-in syllabus," in his highly influential 1967 paper on The significance of learner errors.

Selinker distinguishes interlanguage (IL) from native language (NL) and target language (TL).  He thus emphasises the notion of an independent system, separate from both the first language and the target language (see second language for current terminology). 

An interlanguage is a learner grammar, a mental grammar . . . a natural language system (White, 2003), and as such,

The learner's system is worthy of study in its own right, and not just as a degenerate form of the target system.

(Bley-Vroman, 1983)

Interlanguages are characterized by both marked variation and also great systematicity (see developmental sequences), both within and across learners and with respect to time.

Activity

Classroom illustration: Noticing