Inter-language is the term for a dynamic, rule-based linguistic system that has been developed by a learner of a second language (or L2) who has not yet reached proficiency. A learner's inter-language preserves some features of their first language (or L1), and can also overgeneralize some L2 writing and speaking rules. These two characteristics of an inter-language result in the system's unique linguistic organization.
An inter-language is idiosyncratically based on the learners' experiences with the L2. It can "fossilize", or cease developing, in any of its developmental stages. The inter-language rules are claimed to be shaped by several factors, including L1-transfer, previous learning strategies, strategies of L2 acquisition (i.e., simplification), L2 communication strategies (i.e., circumlocution), and overgeneralization of L2 language patterns.
Inter-language is based on the theory that there is a dormant psychological framework in the human brain that is activated when one attempts to learn a second language. Inter-language theory is often credited to Larry Selinker, who coined the terms "inter-language" and "fossilization." Uriel Weinreich is credited with providing the foundational information that was the basis of Selinker's research. Selinker (1972) noted that in a given situation, the utterances produced by a learner are different from those native speakers would produce had they attempted to convey the same meaning. This comparison suggests the existence of a separate linguistic system. This system can be observed when studying the utterances of the learner who attempts to produce meaning in their L2 speech; it is not seen when that same learner performs form-focused tasks, such as oral drills in a classroom.
Inter-language can be variable across different contexts; for example, it may be more accurate, complex and fluent in one domain than in another.