An interlanguage is an idiolect which has been developed by a learner of a second language (L2) which preserves some features of their first language (L1) and can overgeneralize some L2 writing and speaking rules. These two characteristics give an interlanguage its unique linguistic organization. It is idiosyncratically based on the learner's experiences with L2. An interlanguage can fossilize, or cease developing, in any of its developmental stages. It is claimed that several factors shape interlanguage rules, including L1 transfer, previous learning strategies, strategies of L2 acquisition, L2 communication strategies, and the overgeneralization of L2 language patterns.

It is based on interlanguage theory, which posits that a dormant psychological framework in the human brain is activated with study of a second language. The theory is credited to Larry Selinker, who coined the terms interlanguage and fossilization. Uriel Weinreich is credited with providing the basis for Selinker's research. Selinker noted in 1972 that in a given situation, the utterances of a learner differ from those of a native speaker to convey an identical meaning. This comparison suggests a separate linguistic system, which can be observed in the utterances of a learner attempting to convey meaning in L2. It is not seen when that the learner performs form-focused tasks, such as oral drills in a classroom.

Interlanguage can vary in different contexts, and may be more accurate, complex, and fluent in one domain than in another. A learner's interlanguage utterances may be compared with two things: utterances in L1 to convey the message produced by the learner, and utterances by a native speaker of L2 to convey the same message. An interlanguage perspective may be used to view a learner's underlying knowledge of the target-language sound system (interlanguage phonology), grammar (morphology and syntax), vocabulary (lexicon), and linguistic norms (interlanguage pragmatics). By describing how learner language conforms to universal linguistic norms, interlanguage research has contributed to the understanding of linguistic universals in second-language acquisition.